David Cameron doesn’t do regret. It is not in his nature to sit and fret about decisions that he has taken and can now do nothing about. But there are still a few things that rankle with him. One of those is the House of Commons’ rejection of military action in Syria two years ago.

This defeat was a personal and a political humiliation for Cameron. For months, he had been pushing for action against Assad. President Obama had finally accepted that something must be done following the Syrian regime’s use of chemical weapons. But then Cameron’s own parliament and party stopped him. It sent a message to the world about Cameron, as well as one about Britain and its foreign policy after Iraq.

Downing Street never got over it. In September, George Osborne denounced the decision not to help the rebels fighting Assad as ‘one of the worst decisions the House of Commons has ever made’.

The legacy of this vote complicates Syrian matters today. There’s the task of hitting Islamic State in north-east Syria, a part of the country out of Assad’s control. The military wants to end the absurdity of only hitting the terrorist group on one side of the Iraq-Syria border. Indeed, given that its Iraqi operation is resupplied from its headquarters in Syria, it makes no sense to limit bombing to Iraq.

But No. 10 is reluctant to make it explicit that it is not talking about bombing Assad. This is for two reasons. First of all, Cameron is genuinely revolted by Assad’s behaviour. He believes that someone who is willing to drop barrel bombs on his own people can’t be the answer to the question of what to do about Syria. Secondly, Downing Street doesn’t want to lose face or draw attention to how much its position has changed since 2013.

Since May, the government has been keen to target Islamic State in Syria. But three things held up seeking parliamentary approval. First, the Tories didn’t want the vote to get caught up the Labour leadership contest, and so decided to hold off until the new leader was in place. When, to their surprise, that new leader was Jeremy Corbyn, it scuppered any prospect of the ‘consensus’ on military action that Cameron is so keen on — his guarantee that he won’t lose in the Commons again on a matter of war and peace. Even after Corbyn’s victory, the Tory vote-counters thought they were making progress. They were whittling down the number of Tory rebels and gaining cast-iron assurances from Labour MPs that they would back military action.

But then came Moscow’s decision to deploy major forces to Syria. This, worryingly, took the government by surprise. It’s another example of the UK’s dire lack of intelligence about Russian military action (the seizure of Crimea was not anticipated either). Russian involvement in Syria has hugely complicated efforts to stitch together a Commons majority for bombing in Syria.

Inside government, though, they maintain that they are still trying to build a Commons coalition in favour of action. One of those involved tells me irritably: ‘A majority isnot there at the moment, or we would have a vote by now.’

In fact, a vote is still so far away that there has been no discussion with the whips about the precise wording of any motion. There is also a fear that those Labour MPs who say they are prepared to back it may not be so solid. One of those tasked with courting them says: ‘You have to be able to look them in the eyes and definitely know they will be with you on the night.’ There is a fear that this support could slip away if the question becomes too party political.

A further complicating factor is the American attitude. Washington, in an act of cynicism that has shocked even hard–bitten realists, want to let the Russians stew a bit in Syria. The US is urging an ‘exercise in strategic patience’ to see how deep a hole the Russians will dig for themselves there. Obama is seemingly happy to see Russia dragged in deeper and deeper on the basis that Vladimir Putin will ultimately have to make a humiliating retreat from backing Assad. The US view is summed up to me as: ‘The Russians have bitten off more than they can chew; it’s more like Afghanistan than anything else.’

President Obama is not an easy ally to have. He has no grand strategy, his approach is hard to read and he tends not to return calls at crucial times. One senior government figure says his foreign policy is characterised by ‘indifference and retreat’.

But whatever gripes Whitehall might have about Washington, one fact can’t be avoided: the consensus in Britain about foreign policy has broken down. Labour, still in trauma about the Iraq war, is now a non-interventionist party. When asked during the leadership election, Corbyn couldn’t think of a circumstance in which he would deploy British forces abroad. This makes it very hard to see how the Corbyn-led Labour party would ever support a parliamentary motion approving the use of force. So any such Commons vote will be extremely tight.

The anti-interventionist mood extends beyond the Labour party. After all, if Corbyn and Labour were the only bar to bombing in Syria, Cameron could simply use the Tories’ parliamentary majority to gain approval for it. There is now, though, a strand of Tory thinking that believes western intervention in the Middle East does more harm than good and that the area is best left alone. This bloc of opinion, combined with Labour’s anti-interventionist position, will make it very hard for any Prime Minister without a thumping majority to win parliamentary approval for the use of force in that part of the world.

Tony Blair offered the Commons avote before British forces entered Iraq in 2003 as part of his effort to persuade MPs to back the invasion. But the legacy of that decision and the war itself is that no British Prime Minister will ever again have the freedom of action that he once had in matters of war and peace.