The attack on Nairobi shows how difficult it is to counter the threat of Islamist violence

The result of foreign interventions, aimed at halting an Islamist takeover, has been disastrous

The Islamist militants who have struck Nairobi with such terrifying brutality could not have chosen a more appropriate target than the upmarket Westgate shopping centre.

Kenya is the West’s closest ally in East Africa, and the Westgate mall a haunt of Western expatriates who mingle in the aisles with the beneficiaries of Kenya’s economic boom. It is tragic but not surprising, therefore, that the dead include both Britons and relatives of the President of Kenya. The fact that the terrorists are reported also to have executed children, because they could not name the Prophet’s mother, shows what we are up against. It is pointless to imagine any sort of accommodation with these killers.

At the same time, those who itch for a Western-led military response to terrorist atrocities of this kind will be hard put to define the right response to this outrage. The home country of the al-Shabaab militants who claim responsibility for the attack, Somalia, has already been an object of two foreign interventions. The first was led by Ethiopia, the second by the African Union, which drove the local Islamists from the capital, Mogadishu.

The result of these foreign interventions, aimed at halting an Islamist takeover, has been disastrous. As in Afghanistan, expelling Islamists from the capital achieved little and may have made matters worse. The Somali Islamists did not melt away. They merely felt spurred into committing greater, more random, acts of violence, and unleashed terror on those foreign countries that they blamed for their reverses at home.

Movements like al-Shabaab, whose name, “The youth,” is as vague as its objectives, now appear protean, taking on any amount of forms and upping their activities in one country as suddenly and unpredictably as they lie low in another. That fact alone should remind us of the redundancy of any Western policy towards al-Qaeda and its affiliates that assumes the existence of a single nerve centre or source of authority. These movements can no more be decapitated than a virus, and, as we now see, the much trumpeted execution of Osama bin Laden, however gratifying to the Americans, was in strategic terms irrelevant.

Painful though it would be for many to admit, it is high time that we abandoned the idea that we can shape the outcome of the various civil and religious conflicts raging in Muslim countries by the judicious dropping of bombs or selection of favourites. In Afghanistan it has left us propping up the hopeless, despised President Karzai. In Somalia, via the African Union, we prop up an even more corrupt collection of warlords. Better that we had stayed out of these conflicts militarily. As Kenya mourns its dead, it will have to decide its response to the men of violence. The lesson of recent years is that vigilance at home, not intervention abroad, is the better bet.