15 September 2014 Last updated at 00:05

David Haines: How will the UK respond to IS militants?

By Chris Mason Political correspondent, BBC News

A small group of Conservative MPs went to see both the prime minister and the foreign secretary last week to make the case for the UK to bomb Islamic State (IS) militants.

They feared then what we know now: that a British hostage, David Haines, would be murdered and a video of his killing would be posted online.

For some of those arguing for the UK to join the United States in air strikes against the militants, there was a moral imperative to act.

A failure to do so would be a sign of weakness.

So how will the British government respond to what has happened?

War-weariness

Don't expect a knee-jerk reaction. The approach, as one official put to me, would be "methodical". Another said it would be "resilient".

In other words, they are not rushing. Why?

They have known about British hostages taken by IS for months - even though their plight has only become public in recent weeks.

So the arguments we have heard for some time from the prime minister and others have been made in the full knowledge that what has happened this weekend was possible, even likely.

Secondly, those in government are well aware there is a war-weariness among many in the UK. For many, the long shadow cast by the Iraq war in 2003 has never gone away.

Add to that, a more recent bruising experience for David Cameron: the government's defeat in the Commons just over a year ago over potential military action in Syria.

It means this time around he is worried about the arithmetic: will enough MPs support him, if military action is suggested?

So there is a bit of previous where military conflicts in Iraq and Syria are concerned.

International coalition

The strategy is to assemble, or be a part of, two coalitions.

If UK air strikes are to happen, firstly the government will require a domestic coalition by persuading parliament of the case for military involvement.

MPs on the whole do appear more persuadable than a year ago.

Labour says it is listening to the government's case and it has not ruled anything out.

But party leaders would like to see more detail.

Meanwhile, the government wants to be a part of an international coalition: supporting the Iraqi government and the Kurds and standing alongside other willing volunteers, not least the United States, some Arab states and also Australia.

Officials point out this is already happening, with humanitarian support and RAF surveillance flights over Iraq.

Those who are convinced that UK air strikes are a necessary next stage point to what they see as a very different situation from a year ago and that defeat for the government over Syria.

This time there are British hostages, an IS militant with a British accent is the murderer and there is a fear British jihadists who manage to return home could attempt attacks here.

Downing Street is well aware of the gravity of the situation - and has been for some time.

But it is well aware too that the case for British military action has to be methodical and that persuasion takes time.