Can anyone be trusted? That must be the thought of thousands reading the horrific descriptions of the carnage at Fort Hood. The indiscriminate slaughter was carried out by a US army psychiatrist, a man who, with terrible irony, had been treating American soldiers suffering from combat stress. No one had believed he was an extremist. No one suspected that his meticulous clear-out of his belongings days earlier had been anything other than preparation for deployment overseas. And no one had seen his Middle East origins and Muslim faith as any cause for concern. Until today.

Reports that the killer, Major Nidal Malik Hasan, had called out Allahu akbar (“God is great”) moments before opening fire are alarming. They suggest that some perverted religious impulse may have set him on his murderous path. They will prompt questions about the loyalty of many Muslims serving in the US Armed Forces. And they will suggest to some people, including some Muslims, that Islam is somehow incompatible with service as a soldier and citizen of a secular Western state.

None of these doubts has the slightest validity. There are millions of Muslim Americans, and many thousands of them have served in uniform. Religion, especially in America, is constitutionally divorced from public life, and there is nothing in either the military code or in the tenets of Islam that is seen to clash. That is equally true for Muslims serving in the Armed Forces of Britain and other Western nations. The Armed Forces must be open to people of any faith or none. Some Muslims may indeed find it difficult to discharge their duties as soldiers in Iraq, Afghanistan or other countries where they may have to kill fellow Muslims. That conscientious objection, however, is not solely the preserve of Islam. Nor is it a valid reason for disobeying orders. Those who will not serve wherever they are ordered have a simple option: resignation. They have no conceivable justification for violence.

The massacre at Fort Hood, the worst on a US military base, comes only two days after the killing of five British soldiers by an Afghan policeman. Again, the immediate and dangerous reaction is the loss of trust. Suddenly, to many of the Nato troops deployed in Afghanistan, the men they are training or those fighting alongside them against the Taleban are no longer allies but potential terrorist murderers. And the more devout the Afghan comrade appears, the more he may now be suspect in some people’s eyes.

History teaches another lesson. Muslims have proved brave and loyal soldiers in the British Army in its many deployments across its former Empire. They served with distinction in the Indian Army. Many were part of the poorly acknowledged Indian contingent serving in the trenches of the First World War. The police who did an effective job in containing the communist insurgency in Malaya were Muslims. The Palestine Police did an honourable job in difficult Mandate days.

It is not religion but fanaticism that is the insidious threat. And fanatics, driven by inner demons, are found anywhere and everywhere — from Dunblane to Columbine, Waco to Virginia Tech. Guarding against stress, shock and anger is particularly challenging in the Armed Services: a resort to violence is all too easy. The point of training is to teach humans how to cope with the extremes of fear, danger and pain. Only then will a soldier emerge capable of rational behaviour in battle, or of responsibility towards his comrades. Britain needs to continue the training programme of the Afghan Army and police. The US needs still to prepare the thousands of soldiers at Fort Hood and other bases for deployment abroad. The horrors of the past week should not deflect attention from this purpose.