Copied from a post of mine on an earlier thread:
We aren’t intervening in Syria because we can’t do it:
Legally – As Z11 says, there is no way a UNSCR would get through.
Politically – Syria is at the heart of the Arab world, physically and culturally, and has ties with Iran, Hamas, Hezbollah etc. It would set the middle east ablaze.
Militarily – Syria has a large and credible military. We haven’t got the numbers. Even the Americans would struggle, given their current commitments. There would be heavy casualties on our side, and western democracies won’t tolerate that for somebody else’s fight.
So overall, it’s realpolitik. All we can do is diplomacy, protest, sanction etc. Just because we can’t intervene everywhere, does that mean we shouldn’t intervene anywhere? Hypocrisy or doing what we can?