In the respected opinion of Jeffrey David Sachs—distinguished Quetelet Professor of Sustainable Development at Columbia University, director of the Earth Institute, and special adviser to the secretary-general of the United Nations—the problem of extreme poverty can be solved. In fact, the problem can be solved "easily." "We have enough on the planet to make sure, easily, that people aren't dying of their poverty. That's the basic truth," he tells me firmly, without a doubt.

It's November 2006, and Sachs has just addressed the General Assembly of the United Nations. His message is straightforward: "Millions of people die every year for the stupid reason they are too poor to stay alive.… That is a plight we can end." Afterward, as the two of us have lunch in the crowded U.N. cafeteria, overlooking New York's East River, he continues: "The basic truth is that for less than a percent of the income of the rich world nobody has to die of poverty on the planet. That's really a powerful truth."

Sachs, 52, is devoting his life to this all-powerful truth. As one exhausted member of his staff explained to me, "It feels like we're running a campaign—all the time."

Day after day, without pausing for air, it seems, Sachs makes one speech after another (as many as three in one day). At the same time, he meets heads of state, holds press conferences, attends symposiums, lobbies government officials and legislators, participates in panel discussions, gives interviews, writes opinion pieces for newspapers and magazines, and connects with anyone, absolutely anyone, who might help him spread the word.

One week in early December, Sachs scheduled three overnight flights in five days. First, after a full day of teaching at Columbia, he flew from New York to Rio de Janeiro, São Paulo, and Brasília for two days of meetings with President Luiz Inacio Lula da Silva's Cabinet. From there he headed to Washington to attend the White House Summit on Malaria, hosted by President and Mrs. Bush. Afterward he left for San Francisco, where he made a presentation to the founders of Google. That same day, a Friday, he flew home to New York. Over the weekend he attended a dinner with Ban Ki-moon, the incoming secretary-general of the United Nations. As far as I can tell, the only time Sachs slows down is when he sleeps, never more than four or five hours a night. His wife, Sonia Ehrlich, a pediatrician and the mother of his three children, has been quoted saying (more than once), "I'm a happily married single parent."

According to Sachs, his job is to be "a pest." Bono, who wrote the foreword to Sachs's best-selling book, The End of Poverty, makes the same point, more or less poetically: "He's an irritant," Bono told me, paying Sachs a compliment. "He's the squeaky wheel that roars."

Mark Malloch Brown, who was deputy secretary-general of the United Nations under Kofi Annan, described Sachs to me as "this magnificent battering ram." In unadorned English he added, not without respect, "He's a bully. For the record, he's a bully."

Never mind. To Sachs, the end of poverty justifies the means. By hook or by crook, relentlessly, he has done more than anyone else to move the issue of global poverty into the mainstream—to force the developed world to consider his utopian thesis: with enough focus, enough determination, and, especially, enough money, extreme poverty can finally be eradicated.

Once, when I asked what kept him going at this frenzied pace, he snapped back, "If you haven't noticed, people are dying. It's an emergency."

I had noticed. It's a Sunday in mid-January, and I'm in sub-Saharan Africa. A few of us have trekked to Ruhiira, an isolated village in the highlands of southwestern Uganda. Having passed the equator some time ago, we're now, according to my map, 20 miles or so from the borders of Rwanda and Tanzania.

There's not much of anything in Ruhiira. No electricity or running water. No roads to speak of. We're in a place of lack, of deprivation, of absence. This is the dead land. The soil, once rich and fertile, is utterly depleted from years of abuse. The surrounding hills have been plundered, stripped bare of trees. With no firewood at hand, villagers are forced to dig up banana rootstalks to use as cooking fuel. Matoke, a green starch banana that people boil and then mash, is the staple in these parts; it's about the only thing that grows freely. You won't starve on matoke, I'm told, but you certainly won't thrive. In Ruhiira, 4 in every 10 children are chronically malnourished; their growth has been stunted.

Unsteadily, we make our way down a long and steep and narrow footpath—loose dirt and small stones. At the bottom of the hill we come upon the village's main water supply: a stagnant, filthy water hole with bugs floating on the surface. Women in bare feet, with babies strapped to their backs, bend over to fill plastic buckets and jerricans. Some of the women wear sarongs. Others are dressed in ankle-length gomesi, the traditional dress of Uganda, with high puffed sleeves and wide sashes.

Young children too are helping to collect water. A few of the smallest girls, incongruously, are dressed in torn party dresses, pink, with ruffles, that might have been collected by, say, a church in Tulsa, Oklahoma. I notice one young boy's badly swollen feet: they're a sign of a medical condition known as kwashiorkor, or severe protein deficiency. It's what happens when someone lives on bananas alone, a doctor in our group informs me.

Hunger won't kill these children, despite appearances. Instead, they'll most likely die of malaria. One day they'll fall into a malaria coma—fever, convulsions—and never come out of it. For African children under five years of age, malaria is the No. 1 cause of death. In Ruhiira, it's endemic.

More and more observers arrive; one after the other they scramble down the footpath to get a good look at the women and children standing beside the cesspool. A dozen men wearing brand-new United Nations caps join us. Behind them, snapping photo after photo, is a graduate student from Germany, a sunburned woman in an emerald-green muumuu.

Plenty of journalists have also gathered around the water hole. Over there, that way, being filmed for the BBC, and using Ruhiira's contaminated water as a colorful and authentic backdrop, is George Osborne, a member of Britain's Parliament and a rising star in the Conservative Party. "We're here at the only water source for the village," he intones, looking right into the camera. "And as you can see, the mothers there, some of whom are pregnant, are picking up water which they've then got to take up the hill."

Still more spectators arrive. I meet four sincere, nice-looking Canadian men, square-jawed and blond: Ryan, Tyler, Joel, and John. They're volunteers with a Christian mission whose aim is to bring clean water to villages in the area. "What's going on?" asks Tyler.

What's going on today, in a nutshell, is Jeffrey Sachs: he's the reason we're here in Ruhiira staring at women and children doing what they do every day whether we're here or not—collecting dirty water in jerricans and plastic pails, and carrying it up the hill.

About a year ago, Sachs named Ruhiira a "Millennium Village," one of 79 villages in 10 African countries where his controversial theories on ending extreme poverty are being tested. He approaches alleviating poverty as if it were a rigorous scientific experiment, allocating exactly $110 per person each year for five years to implement a prescribed set of basic "interventions": fertilizer and high-yield seeds, clean water, rudimentary health care, basic education, mosquito bed nets, and a communication link to the outside world. The results are tested and monitored, his goal being to prove that the same scientific model can be used on a grand scale to save the lives of hundreds of millions of people trapped by poverty.

The first of Sachs's Millennium Villages was in Sauri, Kenya, where intervention began almost three years ago. Since then, production of maize in Sauri has more than tripled, while the incidence of malaria in the village has fallen by two-thirds. As well, lured perhaps by the free school lunches, more children than ever are attending the Bar Sauri Primary School. These are the sorts of results Sachs hopes to replicate all across sub-Saharan Africa, starting first in villages and countries that are relatively stable, receptive to change, and eager to work with him.

One of Sachs's biggest supporters is the financier and philanthropist George Soros, who recently donated $50 million to the Millennium Villages Project. (The project is a partnership among the U.N., Columbia, and Sachs's own nonprofit organization, Millennium Promise.) According to Soros, whose foundation gives away between $350 million and $400 million a year, investing in Sachs offered an attractive "risk-reward ratio." "Even though it's a large amount of money, $50 million, I thought there was really little downside," Soros told me. "As a humanitarian action, it was a good investment on its own But if it succeeded, then of course you would get a reward that would be way out of proportion to the investment made."

In short, Ruhiira is a kind of petri dish in the laboratory of Jeff Sachs. And here today, at the center of this tableau, is Sachs himself, standing among the water gatherers of Ruhiira. Wearing a pale-blue dress shirt, he squints awkwardly, uncomfortably, in the sunlight. His head, with its thick sandy-brown hair, seems unusually large for his slight frame. As usual, he's badly shaven. The crowd hushes respectfully.

"Thank you for bringing us to this place," he begins, addressing the villagers off the top of his head, without notes. "We are honored that you have taken us into your community."

His deep midwestern voice is resonant, deliberate. "We have seen how we can work with you to improve the agriculture, with new crops and ideas to improve your income." A translator repeats his words to the crowd in the local Bantu language, Runyankole.

"And we have seen the bed nets in your houses. Do you have bed nets in your houses?"

"Yes!"

"All right!" responds Sachs. He's getting fired up now, and his voice gets stronger. "And are they working? Do they help?"

"Yes!"

"We are happy to see that. We went to the school and we saw how the school feeding program has started and we're very proud of what you have done with that. And we went to the health center to see how it is being expanded, with more health workers in the community.

"Why do I mention all these things? Because for every problem you have, there is a solution! We want to help you find the solution!"

The people clap. Then they begin to cheer. Sachs is pleased with himself and he grins. Now, in a traditional Ugandan gesture that is the equivalent of a standing ovation, the villagers, all of them, stretch out their hands toward Sachs and begin wiggling their fingers. Everywhere you look, like the gentle rain from heaven, fingers wiggle and flutter. The people of Ruhiira are raining blessings on Jeff Sachs, the merciful one.

For many years, during the 1980s and 1990s, Sachs was known as "Dr. Shock," the brilliant macro-economist from Harvard who prescribed radical fiscal and monetary discipline, so-called shock therapy, to countries emerging from Communism. These days, he's better known glibly in the media as "Bono's guru" and as the professor in MTV's masterful documentary The Diary of Angelina Jolie and Dr. Jeffrey Sachs in Africa. In the movie, Jolie calls him "one of the smartest people in the world."

When it was released two years ago, Sachs's latest book, The End of Poverty, was excerpted for a cover story in Time magazine. It also made The New York Times best-seller list; more than 230,000 copies have been sold in the United States, an extraordinary accomplishment for what can be, truthfully, a dreary slog with only charts and graphs for company.

In some of his fine-tuned speeches, Sachs presents his audience with an ethical choice: "Either you decide to leave people to die or you decide to do something about it." Who in the world can resist that call to action? After all, one billion people on the planet are scraping by, barely, on less than a dollar a day. Industrialization has passed them by. They have not been lifted out of poverty by what proponents of free markets like to call "the rising tide." For Sachs, the way to end extreme poverty is obvious; his one question is, How long will it take the rest of us to come around?

"Have you seen children dying?" he asks his audience. We're in Montreal, at an all-day conference devoted to poverty. Bill Clinton will be speaking later in the day. So will Mia Farrow. But, for now, above Sachs's head, projected on a giant screen, is a photograph he took a few months ago at Zomba Central Hospital, in Malawi. Row after row of small children in malaria comas are lying on the bare floor, their yellow eyes rolled back.

"I never thought in the 21st century, growing up in the 20th century, I'd ever see that," Sachs exclaims, outraged by the shortsightedness implicit in that photograph. "Lack of a bed net. Lack of a dollar medicine. Lack of an oral rehydration solution in time to save a child dehydrated from a diarrheal infection. Lack of antibiotics to cure a child of acute lower respiratory infection contracted from living in a hut where dung is burned to cook the meals in a smoke-filled chamber."

His catalogue goes on: "Lack of a five-cent immunization, so that you have hundreds of thousands of children dying of vaccine-preventable diseases. Half a million mothers dying in childbirth because there's no obstetrician or even emergency care to stop the hemorrhaging, to deliver a child in breech, to do a C-section. The most straightforward things that we've known how to do for centuries … does change come about? A few days later, in Nairobi, I meet Charity Ngilu, Kenya's dynamic minister for health. When she took office, in 2002, her priority was to somehow contain the fast-moving epidemics of aids, tuberculosis, and malaria that were ravaging the country. But Kenya faced severe shortages: of doctors and nurses, of medicines, and of such basic supplies as surgical gloves, IV fluids, even hospital food. The health-care system—exhausted, chronically underfunded—had collapsed.

That's when and where Sachs came in. Passionately, he argued Ngilu's case to the World Bank, to the International Monetary Fund, to major foreign-aid donors, and to Kenya's bureaucrats themselves. As a result of his and others' determined work on her behalf, Ngilu attests, Kenya's health budget, while still bare-bones, was increased 20 percent last year and another 45 percent this year. In the past two years, Kenya has managed to hire an additional 3,018 health-care workers and the government recently distributed 3.4 million insecticide-treated bed nets. Meanwhile, new cases of H.I.V./ aids have fallen even as the number of patients receiving anti-retroviral treatment has sharply increased.

"If it wasn't for Professor Jeffrey Sachs, we would not have moved forward," Ngilu states, when we meet at her office in Nairobi. "Those people who are on treatment would still be dying. Those children who are under bed nets would be dead. Women would not be accessing care." Pausing, she shakes her head as if imagining her job without the good professor's help: "The support he has given me!"

Paul Farmer, the renowned medical doctor and humanitarian, whose organization, Partners in Health, cares for people in the poorest, most godforsaken corners of the world, explained to me, "Just five years ago, people like me who were trying to take care of the destitute sick with diseases like aids, we had almost nobody on our side. We had everyone saying, 'It's not doable, it's too complicated, you need a health infrastructure, it's not sustainable.' Then Jeff got involved in this and said, 'Buck up, stop whining, and start getting work done.'"

One of Sachs's most significant contributions to the cause of ending world poverty is a gigantic report, published by the World Health Organization in 2001 and titled Macroeconomics and Health: Investing in Health for Economic Development.

The W.H.O. report lays out the facts in stark terms. Every day, 22,000 people on the planet die of poverty. Spending money on health care in the world's poorest countries is more than a humanitarian imperative, Sachs's report argues; it is also the key to driving economic growth. Co-opting the rhetoric of corporate America, cunningly, the report manages to transform a health catastrophe into a business proposition: saving lives can offer huge returns to investors. With an annual investment of $66 billion, the report states, we could be saving eight million lives a year and generating economic benefits worth $360 billion a year.

In the skilled hands of Jeff Sachs, macro-economist, such gigantic, almost unimaginable figures are made to sound reasonable, even modest. "He's not embarrassed by large numbers. And he's not apologetic for large numbers," said Richard Feachem, who served on the commission for Sachs's report and recently stepped down as the executive director of the Geneva-based Global Fund to Fight aids, Tuberculosis and Malaria. "What he's saying is 'If it needs billions for health and development, don't be ashamed to ask for it.' And, by the way, to anyone who says, 'Oh, that's a lot of money,' say, 'Well, by whose standards?' because by the standards of military expenditure it's not a lot of money."

The total annual sum spent on health care in sub-Saharan Africa is typically $20 per person or less. To put that into perspective, in the United States we spend about $6,000 per person each year on health care.

In Ruhiira, where TB and malaria are rampant and, according to unicef, where one in 13 women will die during pregnancy or in childbirth (the odds are one in 2,500 in the United States), there really is no health care to speak of. The closest hospital is three to four hours away by wheelbarrow, the vehicle most often used to transport the sick from place to place.

I visit the hospital with Sachs. Located 20 miles off the national electric grid, the Kabuiyanda Health Centre has no power or running water. At one time, for a short period, two solar panels had been mounted on the roof. They were stolen. As for the 19-kilowatt generator parked outside the building like a totem, there's not enough money in the budget for fuel.

Without electric power, how do you provide standard medical treatment to people who are dying? Without running water, how do you sterilize surgical tools and wash the blood from floors and beds and open wounds? How do you keep your hands clean or refrigerate medicine and vaccines? As we make our way through the hospital, Sachs looks distraught.

"How many beds are there here?" he asks the young doctor on staff, Stephen Mucunguzi.

"Twenty-eight."

"Twenty-eight beds for 125,000 people?" repeats Sachs, trying to grasp the implication of those figures. "Aren't they filled, filled, filled?"

Dr. Mucunguzi leads us to the operating theater, a plain cement room built in 2002. For several reasons it has never been used for surgery. First of all, it took three years for surgical equipment to arrive after it was ordered. Then, just after the equipment arrived, the only doctor on staff quit, and for almost five months the hospital had no doctor at all. Finally, in late December 2006, Dr. Mucunguzi accepted the job, but only after Sachs's Millennium Villages Project offered to supplement his official $315-a-month salary.

Further problems have plagued the hospital. So shoddy was the original construction of the operating theater that, until repairs are carried out, it cannot be used for general surgery. "We are hoping it will be working in a month," says Dr. Mucunguzi.

Sachs looks skeptical. "And running water?" he asks.

"Well, we plan to put in a water tank. We need a maximum of one month to improve the system."

"So," says Sachs, questioning the young doctor, "today is January 14. Could we really try to have this working by March 1? No later."

"Yes, yes."

"I think it would be good for us to have a goal."

That evening, at dinner with Dr. William Nyehangane, the district's health officer, Sachs discovers that the total annual budget for health care in the area that includes Ruhiira is only $1.90 per person. "Unbelievable!" shouts Sachs. "Unbelievable!

"Did you hear that?" he asks no one in particular. "One dollar and 90 cents. One dollar and 90 cents. Unbelievable."

As a young child growing up in Oak Park, Michigan, Jeff Sachs had a preternatural mind. At 12 or 13 years of age, in middle school, he won a mathematics contest for gifted children, with the result that he spent his summer taking college-level math courses at Oakland University, in Rochester, Michigan. One time, not uncharacteristically, when a high-school teacher assigned a 5-page essay, Sachs handed in 40 pages. "He never had a rebellious day in his life," according to his sister, Andrea Sachs.

You will not be surprised to hear that Jeff Sachs was named class valedictorian when he graduated, in 1972. Nothing less was expected of him, apparently. "His father was extremely bright and was top of his class. We just assumed our children would be the same," his mother, Joan Sachs, told me.

Jeff Sachs's father, Theodore, was a legend in Detroit. A labor and constitutional lawyer who successfully argued several cases before the U.S. Supreme Court (including Scholle v. Hare, in 1962, which helped establish the principle of "one man, one vote" for legislative apportionment), Ted Sachs was said to have one of his generation's finest legal minds. He was stunning in the courtroom, and he was admired for his deep commitment to social justice. "It was his primary goal to do good for others, and he did," Joan Sachs said of her husband, who died in 2001.

It was taken for granted that Jeff Sachs would attend his father's alma mater, the University of Michigan, and that he too would become a lawyer. In the worst case, his family imagined, he'd become a medical doctor. Instead, when he was 17 years old Sachs left Oak Park to study economics at Harvard.

Martin Feldstein, the well-known economist and a longtime professor at Harvard, remembers meeting Sachs for the first time. "I was teaching the graduate macro-economics course," Feldstein recalled. "And he came along—remember, he's a second-year undergraduate, so he's about 19 years old—and he says, 'Well, I'd like to take your course.'" Warning Sachs that he was an unforgiving and demanding teacher, Feldstein discouraged him and advised the young man to stay away from trouble. "I'll take my chances," replied Sachs.

Sachs received an A in Feldstein's class, then stayed on at Harvard for graduate school. A scant three years after being awarded his Ph.D. in economics, with a focus on international macro-economics, he was granted tenure and made a full professor at the university. It was 1983, and he was 28 years old.

It was during his freshman year at Harvard, at a screening of The Sorrow and the Pity, Marcel Ophüls's four-hour documentary, that Sachs met his future wife, Sonia Ehrlich. She quickly got a sense of his single-mindedness. "In the beginning, Jeff would say, 'Wait until I finish my undergrad thesis,'" Ehrlich once told The Boston Globe, describing her husband's promise to eventually slow down. "Then it was 'Wait until I get my PhD thesis' and 'Wait until I get tenured.' Then it was 'Wait until I finish my first book.' Then Bolivia came up.

"It really took me a while to realize this was his modus vivendi," she concluded. "I stopped waiting and began enjoying the positive."

In 1985, Sachs found himself in the Andean mountains of La Paz, Bolivia, acting as an adviser to the country's president, Victor Páz. Desperately poor and chaotic, Bolivia, with its then annualized inflation rate of 25,000 percent, was spiraling out of control. Sachs identified the root problem: runaway government spending leading to a textbook case of hyperinflation, the likes of which no one had seen since 1923, when Germany's Weimar Republic just kept on printing money.

Consulting academic articles on hyperinflation, and recalling his undergraduate training, Sachs designed an austerity plan to jump-start Bolivia. It called for huge cuts in government spending, massive layoffs of state employees, the end of fixed gasoline prices, a complete overhaul of the tax system, debt cancellation, and, above all, an abrupt shift to a free-market economy.

With its country in disarray, the government of Bolivia followed Sachs's advice. It had few other options.

Sachs's plan for Bolivia actually worked: strict fiscal and monetary discipline quickly lowered the country's annual inflation rate to about 15 percent. "Shock therapy," as the plan was later called (to Sachs's chagrin), would become Sachs's trademark. From Bolivia, he proceeded, in 1989, to Poland. When the so-called Sachs Plan, conceived with his colleague David Lipton, was implemented in Poland, it followed the authors' road map and timetable almost exactly. Slovenia and Mongolia came next.

Sachs, then 35, had become an international star in policy circles; some people even referred to him as the most influential economist since John Maynard Keynes. Then, in the early 1990s, at the government's invitation, he attempted to straighten out Russia's economy.

In hindsight, Sachs was probably naïve. Assuming that his reforms could be imposed on Russia as they had been on Bolivia and Poland, he was defeated by a massively bloated and stubborn economy. Russia was not resuscitated by Sachs's shock therapy; on the contrary, Russia was ravaged while Sachs and his ideas were ignored. The country's state assets were looted, and everything valuable wound up in the hands of a few clever men.

In Sachs's view, his failure to reform the country was due, in his words, to "the triumph of politics over economics." One way or another, Sachs and his Harvard colleagues were widely blamed for Russia's failed transition to capitalism. To the delight of many of Sachs's harshest critics—in particular, liberals who viewed economic shock therapy as coldhearted and mechanical—Russia became the blot on his escutcheon.

When I ask Sachs about his failure in Russia, he becomes agitated, prickly, like a hedgehog: "Do I consider Russia a failure of the West? Yes, definitely. Do I consider it a personal failure? No, I find that absolutely preposterous. I don't understand why somebody doesn't ask Robert Rubin, or ask Dick Cheney, or ask Larry Summers, or ask anybody that actually had power at the time about it." He's had it with this line of questioning: "It's preposterous by now, and tired. And it's tiresome, and it's a tired question, and it's absolutely absurd."

According to his account in The End of Poverty, Sachs's focus on extreme poverty began in 1995, when, for the first time, he traveled to sub-Saharan Africa: "Never, not even in the highlands of Bolivia, where illness is rife, had I confronted so much illness and death." Early in his career, when he was thinking about ways of improving people's lives, Sachs had been convinced of the power of open markets, free trade, deregulation, privatization, and fiscal discipline. Now, perhaps in response to this first trip to Africa, he started to promote benevolent intervention.

Some people believe that Sachs's crusade to wipe out poverty is the direct result of his failure in Russia, that he's atoning for his public errors of judgment and compensating for them. Sachs dismisses that simpleminded theory out of hand. As far as he is concerned, his work in the developing world is not all that different from his earlier work in Bolivia and Poland. In an e-mail, he explains to me that his goal has always been "to take on complex challenges and bring to bear expertise in economics and other disciplines to find workable solutions." What I think he means is this: it doesn't matter whether you're using shock therapy to save a nation's economy or prescribing interventions for a village to save human beings. The messianic pattern is the same.

We're sitting cross-legged under one of the few shade trees in Dertu, a parched, inhospitable stretch of land about 85 miles from the Somali border in Kenya. A group of community leaders has gathered to air their grievances and share their frustrations. The temperature hovers around 100 degrees in the shade. I am offered warm sweet tea with powdered milk.

"Our needs are many," begins one of the men, a tall Somali wearing an embroidered kufi. "We suffered through the drought," continues someone else. "We lost many animals, even our donkey. And now the flood has caused even more problems. The little we had has been washed away by the rains."

Of all Jeff Sachs's 79 Millennium Villages, Dertu, a sprawling settlement in Kenya's wretched North Eastern Province, may be the most challenging. The place is marked by catastrophe: drought, famine, floods, pestilence, tribulation—biblical woes. "It is only God and us who know the kinds of problems we have had here," says Sahalan Badi.

One year ago, during the five-year drought that affected the Horn of Africa, the nomadic herders of this region were forced to walk for hours, sometimes days, in search of water. Even their camels were dying.

At last the rains came, in October 2006, a drop or two at first, afterward the deluge. Rushing to save themselves from the floodwaters, Sahalan Badi and her family lost everything they had, which, God knows, was little enough to begin with.

Now, using basic materials donated by Sachs's Millennium Villages Project and by unicef, the people of Dertu are learning to dig and build their own pit latrines. As well, in the hope of encouraging the business of trading camels and cattle, the project has funded the Dertu Millennium Livestock Market, whose long-term goal is for the settlement to keep itself out of poverty and, if things go well, to move up a rung on the economic ladder. The Millennium Villages Project aims to teach people self-sufficiency.

At the same time, problematically, a growing number of households in Dertu have become dependent on international food aid. Month after month, accustomed to the ritual by this time, the people line up for rations: a jug of cooking oil, enriched porridge for children, bags of rice and maize. The local homes—small domed huts made of twigs and held together with ropes of camel leather—are patched with empty grain bags reading, usaid: from the american people.And that's their water hole! And we saw the women there, a pregnant woman, baby on her back, with a jerrican trying to get water out. It was shocking, actually."

Museveni is not so shocked, it seems to me. Or he may be thinking of something else. "Mmmmm."

Sachs outlines his plan of interventions. "My impression, Mr. President, is that this will all happen within one year," he says. "And it shows to me a pretty basic point, which is that … when we're talking about extreme world poverty, it shouldn't take a lot of time to make a difference."

Museveni's support is needed urgently, Sachs wants to say. The situation is dire. People are dying. It's an emergency.

Museveni is interested in the root meaning of the word ruhiira: "Burnt grass, that's what ruhiira means," he informs us, stirring his tea. "That's what ruhiira means."

"Yeah," says Sachs, hurrying to the crucial matter of Uganda's farm productivity. "What we saw in Ruhiira, they're going to get, in maize, six tons per hectare probably. This is really a bumper crop—not just a crop, a bumper crop. And it's because they never had fertilizer before."

Sachs is urging Museveni to launch a nationwide voucher program: offer bags of fertilizer and high-yield seeds to every small-hold farmer in the nation, he suggests. "Go for the big scale," he says dramatically. "Why wait? There's no reason to wait."

Museveni clears his throat. "I use fertilizers once in a while," he remarks, referring to his personal farm, his own situation. "I'm trying to remember: when I grew maize, I harvested 800 bags."

"Eight hundred," repeats Sachs, politely.

"Yes, 800. Eight hundred bags. I must have been using like 50 acres. The bag is 100 kilograms."

"That's 80 tons over 50 acres," says Sachs, running the numbers off the top of his head.

"Mmmmm." Museveni, reaching for the calculator on his desk, starts tapping the keys: "That's 1.6 … "

Sachs is way ahead of him. "Times 2.5 would be … " he says, before concluding, "That would be four tons per hectare."

"Four tons?" asks Museveni, puzzled by the figure.

"Per hectare," repeats Sachs.

"Ah, O.K.," agrees Museveni. "That's what I harvested. Yes."

"You're a master farmer: you got four tons," says Sachs, complimenting the president on his crop yield and anxious to return to the matter at hand. "But the average here is less than a ton," he points out, referring to Uganda. "But with fertilizer you get four tons," Sachs adds, hoping to seize the day. "If you had all the farmers quadrupling their yields, do you know what kind of growth that would mean for this country? That's like a 25 percent increase of G.N.P.!"

Museveni has settled back into his chair. As he sips his sweet tea, his response to Sachs is: "Mmmmm." On the wall directly behind his desk is a single framed photograph, of Museveni.

Later I ask Sachs: what was his impression of the meeting with Museveni? Sachs seems startled, taken aback by my question. Was there any doubt it had been a success? "I thought it was a very good meeting," he answers, with the utmost sincerity.

Nina Munk is a Vanity Fair contributing editor.