Of course there is, as David Cameron asserts, a strong case for military intervention in Iraq.

Abhorrent videos issued by Islamic State militants of British citizens being beheaded help make that case. So do the pleas for help from the Iraqi Government. The plight of communities in Iraq and Syria terrorised by IS fighters are another factor, not to mention the risk of a terrorist Caliphate bordering the Mediterranean.

There is, due to the request from Iraq, legal justification for intervening in that country. But there are major reservations about the military action backed by MPs at Westminster.

Will it be effective? Few observers can see how air strikes - bombings - can easily be used to prevent the beheading of hostages by militant groups. It could help push back forces and support ground troops, but we will not commit ground troops ourselves.

Mr Cameron says he has no intention of putting "boots on the ground" in Iraq, but it seems inevitable someone must.

Neither is it clear how intervention in Iraq alone can stem the threat from IS. Troubled MPs warned Mr Cameron that action will be needed in Syria too, if the danger presented by militant Islamists is to be checked in any meaningful way. He conceded such action would be justified, but is proceeding cautiously - due to a Commons defeat in a vote over attacks on Syria last year.

The SNP's MPs resolved to vote against the action on the basis that without a long-term strategy, mission creep is a danger. The party is right to raise these concerns.

Such a strategy, if it exists, remains opaque. Although he asserted during the Commons debate that a comprehensive plan existed for dealing with the IS threat, Mr Cameron did not clarify it.

The Prime Minister also rightly said it was vital to have a clear idea of what a successful outcome would look like. But he did not deliver this, either.

The tactic appears to be to go into Iraq - because this is the path of least opposition - and proceed from there. The Government appears to be placing a good deal of reliance on Iraqi and Kurdish troops to do the rest. Is this plausible? The Iraqi army failed to check the advance of IS militants - will still further British training help?

We also need to know the Government's exit strategy. The prime minister says UK armed forces could be in for the long haul, potentially for years. If entering an open-ended conflict without a clear script for success or withdrawal was ever acceptable, it is not any more. This is the third time in two decades our military will have intervened in Iraq. The mess the country is in now is of our own making.

Mr Cameron says military action must be part of a "comprehensive" political and humanitarian plan.

What is that plan? Mr Cameron has public support for action at present, as reflected in the Commons vote. But before we can give it unqualified support, we need to be given more detail about how this campaign is envisaged to develop, with a clear timescale and a recognised end point.