It is without modern precedent for a prime minister to lose control of his foreign policy, let alone decisions about peace and war.

That, though, is what has happened in the past 24 hours.

David Cameron summoned MPs to return early from their summer break in order to vote for British involvement in military strikes against Syria within days.

The timetable was not his. It was President Obama's.

Having declared that the use of chemical weapons in Syria would be the crossing of a red line, the White House was under pressure to act and to act fast.

No guarantee

In addition, Obama is said to have wanted to act before leaving the US for a foreign trip next Tuesday.

If he still wants to stick to that timetable, Britain will no longer be with him.

The government simply could not guarantee that its own MPs would give it a majority in the vote tonight. They needed Labour's support.

Ed Miliband showed every sign of offering it in a series of face-to-face meetings with David Cameron until last night, when he insisted that MPs be given a second vote after the UN weapons inspectors in Syria had reported.

David Cameron has consistently talked tough on Syria and consistently proved unable to act tough. Today he will try to present that as a search for consensus after the painful wounds of Iraq.

It is impossible to know whether he was acting out of principle or in response to pressure from his own MPs. The answer is probably both.

In response the prime minister felt he had no choice but to buckle. So, now MPs are being asked to vote on the "principle" of military action but with the promise that they will get another say before any missiles are fired.

Labour are still not happy. They are tabling an amendment which sets out what they call a "road map" to a decision - in effect a series of hurdles that have to be crossed before action can be taken.

The one that could prove trickiest is the one that may seem the easiest. It is the call for "compelling evidence" that chemical weapons were used by the Assad regime.

The UN weapons inspectors in Damascus will not produce that evidence.

Sarin poisoning

Their mandate is not to discover who used chemical weapons. It is to confirm whether they were used.

This morning the government will publish a document (after Iraq, no-one will call it a dossier) written by the Joint Intelligence Committee.

It has already been seen by senior government ministers and the Labour leader and shadow foreign secretary. It is largely an analysis of so-called "open source material", ie YouTube videos of the chemical weapons attack last week.

I am told that the JIC assessment is that:

The sheer number of images could not have been forged

The symptoms of victims are consistent with Sarin poisoning

The scale of attack would require artillery and aircraft available only to the Assad regime and not to the rebels

As with much intelligence, that is an expert judgement but is it what Labour and their allies in other parties regard as "compelling evidence"?

Tonight, MPs will be offered the chance to vote first for Labour's amendment and then for the government's motion.

Lessons of Iraq

Labour will not say whether they will back the government if their amendment falls.

They will hope for further concessions from ministers. The outcome is, after the last 24 hours, unpredictable.

The stakes could scarcely be higher. At issue is not just what response, if any, Britain makes to the use of chemical weapons; not just British involvement, if any, in military strikes but also the reputations of the prime minister and the man who wants to replace him.

David Cameron has consistently talked tough on Syria and consistently proved unable to act tough. Today he will try to present that as a search for consensus after the painful wounds of Iraq.

Ed Miliband is emerging, his supporters hope, as a statesman who has learnt the lessons of Iraq.

However, he will be aware that the Tories are waiting to present him as someone merely playing Westminster politics if he refuses today to take yes for an answer by backing the government's new motion.