In 2003, in the leadup to Iraq, the UK government sought the consent of Parliament to go to war. While aimed at provided Tony Blair PR cover and making the opposition complicit in his decision (thus making it harder for them to criticise it), the vote also established a constitutional convention. That convention was reinforced in 2013, when David Cameron sought Parliamentary approval to bomb Syria, and was voted down. And now, the need for a Parliamentary vote has deterred another bombing campaign:
David Cameron is to shelve plans to call a House of Commons vote to approve British airstrikes against Islamic State (Isis) targets in Syria in a belief that he has failed to win over enough Labour MPs and a recognition that Russia’s military intervention has complicated the picture.
Downing Street has decided that RAF involvement in coalition airstrikes against Isis targets should not be extended from Iraq. There are fears that the prime minister would be weakened on the world stage by a second parliamentary defeat over military action in Syria.
Ministers had been discussing a potential Commons motion with Labour MPs as recently as 10 days ago, but the government has undergone a change of heart after failing to win the support of sufficient Labour MPs to be confident that it can overcome what is likely to be a sizeable Conservative rebellion.
Of course, the US is still bombing Syria (as is Russia), but the UK won't. And stopping them from becoming further entangled in that clusterfuck and making things even worse is a Good Thing.
...which illustrates why we desperately need Parliamentary approval for military deployments here. It would bring democracy to our most important foreign policy decisions, while ensuring that there is in fact majority support for it. But the latter is precisely why successive governments have opposed this change.
