Precedent plays a big role in parliamentary procedures. There was a tradition of Prime Ministers making use of Royal Prerogative powers to lead us into military conflicts, without the prior consent of parliament being required. This is the way that Tony Blair intended to act over the invasion of Iraq, until a campaign led by Graham Allen the MP for Nottingham North helped to change his mind. What happened is explained below in Graham's own words, as they appeared in Hansard. What he did meant that prior to the invasion of Iraq, Blair was pushed into seeking the support of the Commons for his intended actions. Which at least gave the opportunity for large numbers of Labour MPs to oppose the move, even though we failed to block the proposal.
When it came to the proposal to bomb Syria, David Cameron felt obliged to seek the support of the Commons for this probable line of action. The logic of having a coalition Government might have required him to do this anyway, but Graham Allen's precedent also helped considerably. The Prime Minister still enjoys the ability to resort to the use of the relevant Royal Prerogative Power in the future. But yesterday's defeat of the Government's proposals has further helped to ensure that future Government's are likely to feel the need for prior parliamentary permission before they lead us into military action. Well done Graham.
From Hansard of 13 June 2013, here is Graham explanation of the line he pursued in 2003 -
"Mr Graham Allen (Nottingham North) (Lab): I was one of the organisers of the rebellion, and it was with great sadness that I rebelled against my party and my Prime Minister. Will my right hon. Friend concede that the vote was not gifted by the Government, but hard fought for? Many of us worked for many months to obtain the vote. Indeed, there was to be an alternative convening of Parliament in Church House, at which we would have had a critical mass, and only 48 hours before the Government conceded that there would be a vote. We had enough Members to convene a Parliament to discuss the Iraq war, and the former Speaker, Bernard Weatherill, was prepared to chair it. It would have included Members from across the House, including some very brave Conservative Members, Members from the Liberal party and friends from the smaller parties across the political spectrum. But 122 Labour Members voted on the first occasion, and indeed the numbers went up on the second vote, which is unheard of, given the whipping operation against those who did not want us to go to war. It was not a gift of the Government; it was hard fought for ".
Also, see here on the parliamentary arithmetic of yesterday's vote.