Generals are often accused of fighting the last war when they should be fighting the one they are in. So it is with politicians: they often take up the position they wish they had taken the last time a particular question came around.

It is increasingly likely that before Christmas, MPs will be asked to vote on whether the UK should get involved militarily in the war against Isis in Syria. This has happened before. In August 2013, David Cameron put a proposal before Parliament to bomb targets inside Syria, and was humiliatingly defeated. That defeat, by 285 votes to 272, has kept this country out of the Syrian civil war, so far.

Older hands will remember how they voted in March 2003, when Tony Blair sought Parliament’s authority for the Iraq war. Many of those who voted for war that day have come to regret it, for reasons too familiar to need repeating, whereas no one who opposed bombing Syria two years ago is known to have had second thoughts.

David Cameron has therefore been understandably cautious about returning to Parliament to seek agreement to become involved in Syria’s internal conflict. The default position of many MPs, possibly the majority, would doubtless be to stay out of it.

Actually, no matter what Parliament thinks, we are already involved, because British warplanes have been bombing Isis positions in Iraq, with the consent of the Iraqi government. In the minds of those who support the Isis death cult that is probably sufficient provocation to attack a British city in the same way that Paris was attacked on 13 November. Isis is our enemy, as surely as it is France’s.

The hasty, ill-prepared action proposed by Mr Cameron in 2013, which Parliament rightly blocked, would have been directed against the Syrian army, which is fighting against Isis as well as other rebel forces. We would have been weakening our enemy’s enemy. In 2013, and in 2003, there was an alternative to military force, because the enemy was a dictator who, for all his brutality, could be made to respond to outside pressure.

This time, negotiation is not an option. Isis does not negotiate. It has to be defeated. The choice before Parliament is whether the UK contributes to that defeat or leaves all the fighting to others.

That is not to argue that we should send in the bombers without thinking through the consequences. No British action is going to decide the outcome of Syria’s long war, which will be settled on the ground, not in the air. Earlier this month, a thoughtful report by the Commons Foreign Affairs Select Committee insisted that “there should be no extension of British military action into Syria unless there is a coherent international strategy that has a realistic chance of defeating Isis and ending the civil war in Syria”.

That is absolutely right, and may not be beyond the reach of the international community. There were signs that the diplomatic impasse over Syria started to break up when world leaders met in Vienna on the day after the horror in Paris. Mr Cameron has said he will respond to the Foreign Affairs Committee first, and will then decide whether to risk another vote on military action.