In the past three months, 150,000 people have arrived in the Dadaab refugee camp in northern Kenya. About 80% of them are women and children and many have walked 100km to reach the sprawling, crowded camp of 440,000 that now counts as Kenya's third largest city. This is all we can see of Somalia's famine – the ones who manage to get out. Those suffering in Somalia are largely beyond the reach of the television cameras and the aid agencies. This is a catastrophe the world is finding it easy to forget.

Last week, the predicted figures climbed to a staggering 750,000 who could die in Somalia before the end of the year. That's more than double the number who died in the early 1990s in a previous famine, despairs Mark Bradbury, an expert on Somalia at the Rift Valley Institute . He adds that 20 years ago conflict in Somalia was more intense and puts the crucial question, why is the likely death toll so much higher now?

After the famine in Ethiopia in the mid-1980s, there were plenty of declarations that it must never happen again. Since then millions have been invested in famine monitoring and prediction systems. This is a region prone to food insecurity, but the lesson of Ethiopia a quarter of a century ago was that it is not natural disasters, such as drought, that cause famine but human aggravation of them by conflict.

Aid agencies are beating their chests with guilt and anguish. "It's a catastrophic breakdown in the world's collective responsibility to act", declared Oxfam . But the prediction system worked – it warned of the imminent famine a year ago. Moreover the public response to the appeal has been generous – the British government's response in particular. So why is this food crisis likely to become the worst famine ever in Somalia?

The predicted death toll didn't reach the top of television bulletins last week. Attention was focused on the 10th anniversary of 9/11. But what is almost routinely overlooked – except by longtime observers of Somalia – is that its plight is bound up with 9/11 and the way that the war on terror shaped US foreign policy. In fact, when historians reflect on the early decades of the 21st century Somalia, alongside Afghanistan and Iraq, will be seen as having paid a colossal price in human life as a result of the US war on terror. The deaths in Iraq were brought by bombs, the deaths in Somalia are from hunger: both are a direct consequence of the violent extremism triggered by US aggression.

Somalia's catastrophe is about how "humanitarian space" – the principles of neutrality crucial to effective intervention – has been destroyed by US policy in Somalia since 9/11. This is the key difference with the famine of the early 1990s, when the warring clans still recognised the neutrality of humanitarian aid rather than seeing it as a tool of western political strategy. Now the fringe extremist Islamist al-Shabaab militia, who control many parts of Somalia, will not allow access to most western aid agencies; the World Food Programme had to pull out in 2009, cutting off the food aid on which thousands were already dependent. The deaths of aid workers have forced most western aid agencies to withdraw from working in the country. The result is that there is no one who can engineer the massive logistical effort required to provide the food needed.

The hostility of al-Shabaab to western aid is in all the media reports on the famine. It plays easily into stereotypes of senseless and cruel violence in obscure African conflicts. But what is often omitted is any explanation of why al-Shabaab are so hostile to westerners – one honourable exception is the US journalist Jeremy Scahill, who uncovered CIA sites in Mogadishu. His reports trace how al-Shabaab's suspicion is rooted in the experience of a decade of devious US manipulation. Somalia has been the war on terror's sideshow – and I choose the word deliberately: think of Cambodia and its bombing by the US during the Vietnam war.

From the start Somalia was sucked into the war on terror, a hapless bit-part player in a much bigger global drama. The presence of a small number of people with links to al-Qaida was sufficient to provoke US anxieties that the country could become a haven for al-Qaida members fleeing Afghanistan. Soon after 9/11, the US froze the assets of Somalia's biggest remittance agency and a pillar of the economy, al-Barakat, and many lost money. Another US counterterrorism measure criminalised organisations whose support could end up in the hands of those with terrorist links. This has made any negotiations with al-Shabaab over aid to the regions they control very difficult for aid agencies.

Meanwhile, as US troops – and their embedded journalists – invaded Afghanistan and Iraq and dominated the world's attention, the covert operations directed from the US base in Djibouti got under way in the already chaotic Somalia. At frequent points local warlords and governments in the region exaggerated and used the US fear of al-Qaida to serve their own ends. In Somalia the US has assassinated, rendered, interrogated and bombed by drones those it perceived to be its enemies while funding those it believed to be allies to fight proxy wars. Enemies were switched to allies and vice versa in a dizzyingly complex history: the current president, propped up by the west, is a former enemy who was ousted by an Ethiopian invasion backed by the US in 2006.

The roots of al-Shabaab's rise lie in that fatal US miscalculation (backed by the EU) to support with air strikes Ethiopia's occupation of Somalia and remove the relatively moderate Islamic Courts Union, despite it being the only force to have achieved any degree of stability in the country in the past 20 years. Deep historic enmity between Ethiopia and Somalia provided al-Shabaab with the chance to exploit widespread nationalist anger to gain power.

Scahill grimly comments that "a decade of disastrous US policy [has] ultimately strengthened the very threat it officially intended to crush". Even more depressingly, he adds that Obama, far from drawing the war on terror to an end, has intensified its deployment, authorising and "normalising" violent operations wherever the US deems fit with deeply destabilising consequences.

But no aid appeal wants to complicate things with politics, and agencies such as Oxfam prefer to place the emphasis on the worst drought in 60 years rather than get into a tricky controversy criticising the US or UK government; Oxfam is receiving large amounts of UK aid for its operations in the Horn of Africa. Only Médecins Sans Frontières has openly criticised US policy for provoking al-Shabaab's conviction that aid is a western political tool and that it has been used as such in Afghanistan and Iraq.

Sideshows in a war, by definition, are not centre focus either for policymakers or those who might call them to account in the media. With that kind of neglect, policy becomes erratic, often made on the hoof with little discernible strategy or careful analysis. US policy has not only been spectacularly counterproductive, it has also been grossly irresponsible. As a result, the sideshow – as Cambodia discovered with the rise of the Khmer Rouge and its disastrous consequences – can be even more brutal than the main theatres of war. And the tragedy is Somalia's.