@Naomi
//How long for? Forever?//
The worst wars are the ones which drag on and on, to no clear conclusion.
Thanks largely to a computer based wargame series, of all things, I have come across snippets from Sun Tzu (ca. 500BC). One quote which really sunk in was that you should not start wars unless you have a reasonable prospect of not just winning but winning quickly.
Obviously, it is bad to be the defeated nation but, at least, losing quickly means minimal pain and casualties. That is provided that the conqueror is chivalrous and/or wants to keep their captives alive, to work the land. This wasn't always the case, of course but victims of atrocities either never saw it coming or, if they did, had no choice but to fight, to the death, anyway.
At the moment, I don't think we would lose (on their ground) but our manpower and resource levels are such that we wouldn't overwhelm them and it would just drag on, for years, as per Iraq & Afghanistan. Also, we would be aiding and abetting a despot (Assad) who a major business client of ours is not well disposed towards.
We certainly can't engage Assad and IS simultaneously, so I think we should stay out of the fight until a stage is reached when both are weakened from fighting each other. Not at all humanitarian of us but there seems to be no good course of action for us and events like this have, presumably, played out through history. The difference is that with modern mass media, we can't avert our eyes or pretend it isn't happening.
Life is being a bit of a ***t sandwich, at the moment.