I see al lot of parallels between the Spanish Civil War, which to various degrees involved Germany and Italy giving their support to Franco whilst an "international brigade" took sides with the Nationalists who eventually lost convincingly, after ten of thousands were killed either in combat or in multiple atrocities, carried out by both sides.
The main point is that then the League of Nations took no part and the major powers UK, France, USA just watched or tried to enforce a weak naval blockade, or did nothing. Just like the UN et al are doing now.
The whole of the Middle East is a tinder-box and parts have already been set alight and the "fire" is now out of control now and arming the rebels (whoever they are) won't stop the fire spreading or put it out.
The traditional super powers are weaker now in relative terms then they were during the height of the cold war and the threat of intervention is no longer a deterrent in the way it perhaps once was. Emerging powers in the region such as Saudi Arabia and Turkey are not ready to support intervention by a UN-backed coalition and in any event Russia and China have their own agendas and would veto even the introduction of a no-fly zone.
In short, the West is s*****ed! I am afraid IMHO that it will get a lot worse for the people of Syria (on both sides) before the killing and destruction will stop.
MB