By late June and early July, when their goats were all gone and the last of their cows had sunk to their knees and died, the men told their families it was time to leave. In Daynunay, Haji Hassan and his children packed up what they had a few rags, plastic bottles, some old cooking pots and set out for Mogadishu, 250 km to the east. At every village they passed, their small group grew, first to a column of hundreds, then thousands, then tens of thousands, as millions across southern Somalia abandoned their homes. With little water and only leaves to eat, the young and the old quickly perished: one of Hassans grandsons was buried where he dropped. Bagey Ali, 50, who walked 300 km from Qansax Dheere, says he saw seven people just sit down and die. When his children would start fading on the 500-km trek from Baoli, Bishar Abdi Shaith, 60, carried them on his shoulders. When I realised they were dead, I would lift them off and bury them there, on the way. He lost two boys and three girls that way. A mass exodus, an emptying of half a country, is an unprecedented, biblical event. What triggered it? The immediate cause was drought. Rains failed last October in East Africa, then again in April, and by early August the UN was putting the number of people at risk from hunger in Djibouti, Ethiopia, Eritrea, Kenya, Somalia and Uganda at 12.4 million. Southern Somalia was in famine. A full 2.8 million people, 63% of the regions population, were either starving or at risk of it. The number of Somali children with severe acute malnutrition near death was 170,000; 29,000 had already died. Even those cataclysmic figures were probably underestimates. Iffthikar Mohamed, country director for Islamic Relief (which has staff inside the famine area, unlike the UN), said his teams found mortality and malnutrition rates at least twice as high. Senior relief managers tell TIME there is no chance of preventing 100,000 Somalis, perhaps more, from dying in the next few weeks. How did this happen? Could it have been stopped? And how is it that millions of Somalis were so sure that no help was coming that they took their families on a death march across the desert? The answers reveal how a war between militants and the US and its allies led directly to human catastrophe. When I ask Bagey Ali when the last rain fell in Qansax Dheere, he laughs at the idea, then struggles to remember. Two years, he says. Maybe four. Southern Somalia is part of the Sahel, the band of dry land that runs across Africa below the Sahara. Half a century ago, rainfall was sparse, but droughts occurred only once a decade. Today they come every two years, and in areas where El Nio and La Nia disrupt the seasons, there havent been good rains for 10 years. This is climate change now severe and lethal. The UN Food and Agriculture Organisation says this years drought is the worst since 1950-51, and the compound effect of successive rain failures means an area the size of France has become desert in 50 years. But drought just sets the conditions for famine; only man ensures it. The southern US is in drought, but Americans arent starving. Why? Because Americans have enough government and wealth. Likewise, one reason we are not seeing a repeat of the 1984 Ethiopian famine, in which a million people died, is that much of East Africa has progressed since then. Also, aid workers are now better at saving lives. An early-warning system first predicted East African food shortages 11 months ago, food aid has become more sophisticated and includes medicines and high-protein nut pastes, and improved disaster mitigation is matched by better prevention. Schemes like the USs $3.5 billion three-year program Feed the Future push ever more money into projects such as irrigation and food warehouses that raise peoples ability to feed themselves. Its really important we understand the progress even in the face of this tragedy, says Nancy Lindborg, who is leading USAIDs famine response. Such progress only throws the disaster in Somalia into sharper relief, however. Because if it is humans who produce or prevent famine, who made Somalias? The big difference between Somalia and the rest of East Africa is war. Somalis have been fighting one another and have lived without a central government for 20 years. Perhaps a million people have died. One symptom of this lawlessness is piracy. What began as a fight between clan warlords became, in its second decade, a struggle between warlords. The more extremists then formed al-Shabab, or the Youth. For four years, al-Shabab has battled the official Transitional Federal Government (TFG). The US is the key international player. Since the 1993 battle known as Black Hawk Down, when 18 US troops died during an intervention to support a UN mission in an earlier famine and the bodies of two were dragged through the streets, few Americans have set foot in Mogadishu. But Washington pays close attention. Osama bin Laden first shot to the top of the CIAs danger list not on 9/11 but on Aug 7, 1998, when his Somalia-based unit blew up US embassies in Nairobi and Dar es Salaam, Tanzania, killing 230 people. When al-Shabab allied with al-Qaeda, it too found itself in American crosshairs. The US strikes when it can. When Ethiopia invaded Somalia in late 2006 to topple the Islamic government, US Special Operations troops went with them and detained about 20 al-Qaeda suspects. Washington has also assassinated several Islamic leaders in Somalia, using Predator drones, cruise missiles launched from warships in the Indian Ocean and, once, a helicopter gunship. Those efforts are assisted by a CIA station in Mogadishu and US-funded mercenary operations. Also, Washington bankrolls the unelected TFG, which is perhaps best understood as a US attempt to create a Somali leadership whose authority does not depend solely on firepower. In spite of such labors, al-Shabab was ascendant a year ago. It seemed set to take Mogadishu and announced its international debut in July 2010 with twin suicide-bomber attacks in Kampala, Uganda, which killed 76 people. AID AS A WEAPON By then another US initiative was starting to bite. In 2008 the US State Department listed al-Shabab as a foreign terrorist Organisation, making aiding or abetting it a serious crime. Al-Shabab was stealing aid to feed itself and to sell. Theft of aid is a routine occurrence, but when al-Shabab was designated as a terrorist group, it meant that US officials and foreign aid workers whose actions benefited al-Shabab, even unwittingly, would be penalised. By late 2009 the US was withholding about $50 million in food aid from al-Shababs territory in southern Somalia, saying it had no legal alternative. By early 2010 the US was in a standoff with aid workers, requiring them to refuse to pay the tolls al-Shabab demanded if they wanted US funding. For its part, al-Shabab expelled the World Food Programme (WFP) in January 2010, saying food aid created dependence and that the Organisation was an American proxy: 60% of the WFPs food is from the US Al-Shabab also claimed WFP contractors were corrupt; a Western investigator tasked with probing the WFPs operations in southern Somalia tells TIME many contractors were indeed skimming anywhere from 25% to 65% of aid to sell in the market. In effect, southern Somalia was largely without aid and lacked a reliable distribution network through which to move emergency supplies in the event of a disaster. Warning of a crisis, Mark Bowden, UN humanitarian coordinator for Somalia, accused the US of fighting its war with aid. Were no longer involved in a discussion about the practicalities of delivering humanitarian assistance with proper safeguards, he told reporters in February 2010. It had become an issue of where assistance can be provided on political grounds. On its narrow terms, US strategy succeeded. Al-Shabab has been severely weakened by a combination of famine and the loss of Middle Eastern funding since the political turmoil there. The group has suffered desertions and bloody internal divisions over whether to accept aid. On Aug. 6, it withdrew from Mogadishu. But what impoverished al-Shababs few thousand fighters also helped push a few million Somalis to the brink of starvation. The same areas ruled by al-Shabab are those now blighted by famine. On the ground in those regions are the International Committee of the Red Cross, several Islamic charities, a handful of Mdecins sans Frontires workers and local UNICEF contractors. Thats it. The UN says just 20% of the 2.8 million southern Somalis in need are being reached. The WFP, the giant of famine relief whose slogan is fighting hunger worldwide, is absent. Asked whether the US inadvertently contributed to the famine, Bruce Wharton, US Deputy Assistant Secretary of State for African Affairs, chooses his words carefully. US sanctions against al-Shabab do not and have never prohibited the delivery of assistance to Somalia, including those areas under de facto control of al-Shabab, he says. While Wharton is technically correct, the practical effect of US sanctions has been, precisely, to block aid to southern Somalia. Thats something the US implicitly acknowledged in early August when it felt the need to issue new guidance to humanitarians, which, Wharton says, should help clarify that aid workers who are partnering with the US to help save lives under difficult and dangerous conditions are not in conflict with US laws and regulations. Some aid workers openly accuse the US of causing the famine. The famine is proof of US success, says Burns. Washingtons allies in the TFG arent shy, either, about hailing the strategic advantage the disaster gives them. The famine is an opportunity to expand our reach, TFG Prime Minister Abdiweli Mohamed Ali tells TIME. The more weak [al-Shabab is], the more we reach. A TFG military adviser confirmed that the TFG wanted no aid reaching southern Somalia until after it had defeated al-Shabab. Underlining the point, TFG commander General Yusuf Mohammed Siad, better known as Indaade, says, We cannot take food to where they are. They have nothing, they cannot fight, and what we need to do now is clear them out. After that, we will take food there. HELP OUT OF REACH Slowly, hundreds of thousands of starving Somalis are realising they may have trekked to Mogadishu only to die there instead. If theres little aid in the famine areas, theres also not enough outside them. Afraid for their safety, most Western aid agencies hole up in a fortified base next to the airport. Three weeks after the UN declared a famine and despite several aid flights arriving daily, food had yet to make it out of the airport to a camp just 100 m away. Many in the aid world struggle to explain their poor performance. They should have been prepared. The UN has for years described Somalia as the worlds worst humanitarian crisis, and there were drought forecasts a year ago. The funds raised are dangerously inadequate, said UK International Development Secretary Andrew Mitchell after visiting Mogadishu on Aug. 17. That hasnt stopped some aid groups from claiming a heroic success. The WFP broadcast a fundraising Twitter message on Aug. 9 proclaiming, Airlifts launched to bring enough high-energy biscuits to Horn of Africa to feed 1.6 million people. Its all too much for Mogadishus mayor, Mahamud Nur: How many people will die? The aid groups say theyre here, but where? Its complete rubbish Children are dying The day I first visit Banadirs 35-bed childrens ward, a 7-year-old child named Umar has just died. The next day two 1-year-olds follow, a tiny 18-month-old boy dies minutes after we arrive, and Abshir, a 9-year-old who looks 4, dies that evening. On repeat visits, I start recognizing children from the camps, now in sudden decline vomiting, defecating, breathing shallowly, their eyes rolled back in their heads. Banadir has lost its cemetery. Refugees have built huts and a makeshift classroom over the graves. As relatives scour Mogadishu for an unclaimed space big enough in which to lay Umars tiny body, I stand with his mother Khalima, 38, as she watches an orderly wash her sons pin-thin body and wrap him in a white shroud. Overhead, unseen, a Predator drone hums. Then, in the graveyard school next door, the children start to sing. TIME