In his speech before the war in Iraq Tony Blair, the UK Prime Minister, argued that not intervening would send a message that tyrants would feel at liberty to act without consequence. By military action in Iraq, he argued, those who would perpetrate atrocities would think twice. There would not seem to be any evidence, before or since, that would back up that contention.
We now have David Cameron, the current Prime Minister, making essentially the same arguments – do politicians ever learn from history?
In both Iraq, and now Syria, the UK national interest was not clear. There appears to be little direct threat to Britain and our economic interests there are limited. Indeed there seems to be more of a threat from British intervention becoming a recruiting sergeant from homegrown terrorists. So it comes down to a humanitarian argument; since when was Britain (and its allies) the arbiter of international law and by what authority do they seek to be its enforcers? Surely that is what the UN was created to do?
As a final thought I suspect it matters little to the thousands of innocents that died due to chemical attacks any more than it provides comfort to the tens of thousands that they died due to conventional war. Surely the humanitarian argument was given up many months ago in Syria. War of any kind, especially when inflicted indiscriminately, is an atrocity.