THE decision to commit British air forces in the defence of the Iraqi government was taken soberly and carefully last week, by a Parliament in full possession of what facts can be gleaned from a chaotic and fast-changing situation. Several parliamentarians referred to just-war principles, without mentioning their Christian roots, and declared themselves satisfied. Lord Hannay, for example: "Having scrutinised these six conditions - just cause, last resort, legal basis, reasonable prospects, regional support, and proportionality - we believe that they have been met." This is an over-hasty judgement; none the less, the circumstances more resemble the first Gulf War, when speed was of the essence, than the second, when the urgency was manufactured. Enough is known about Islamic State to be sure that, for the present at least, negotiation is out of the question.

As a method of going to war, this step-by-step approach has much to commend it: Parliament has not handed a blank sheet of paper to the Prime Minister, to fill in as he wishes. It can adjust the degree of engagement to match the situation on the ground. Steps forward can be followed, in theory, by steps back. On the other hand, there is a danger that such control is illusory. Each step taken makes a further step easier to contemplate. It is illogical to employ air-strikes without the use of ground forces; it is illogical to confine your action to Iraq when Islamic State operates across the border in Syria. It is possible that the UK will find itself involved in a war of indeterminate length without ever really having agreed to it.

The UK's behaviour requires the utmost vigilance, therefore, not least because the situation in Iraq and Syria is hopelessly complex. As has been seen in Libya, action against an oppressor is pointless unless care is taken to ensure that what follows is not worse. On the other hand, as has been seen in Syria, staying one's hand against an oppressor can lead just as readily to untold misery and death. Difficult decisions have sometimes to be made quickly when not every fact is clear. With Baghdad under threat, hand-wringing about military action was a luxury that the Government could not afford.

A key element in the decision was the involvement of Arab states in the anti-IS coalition. Speaking in the Lords, the Archbishop of Canterbury warned of the escalation of the conflict to include disaffected young Muslims elsewhere. Anything that stops the coalition being represented as a Western imperialist initiative is to be welcomed. He spoke of the need for a vision to counter the one promoted by Islamic extremists, and suggested something from the Judaeo-Christian tradition. But there is surely enough in Islam to counter the indiscriminate murder and rape committed by Islamic State militias. British Muslims have argued as much, and we are, thankfully, beginning to see the same message emerge from Islamic nations in the Middle East.