The Pope himself has called on the world to ‘do something’ about the butchery against Christians by uslimes, implying military or warlike action.
Church teaching doesn't change no matter what a man says. Or woman for that matter.
There is a just war, and the war against these radical militias (of which ISIS is the best known, but not the only one) is by definition just.
Some of the Christians in Syria have been fighting very hard, and giving their lives in the struggle against evil. However, they under sanctions, by the UN and EU. The Western world has supported these militias time and again - that is what needs to stop. Recognised governments, such as of Libya and Syria, have been put down in favor of the rights of insurgents. The Christian did not seem to notice the ill-effects of this, until it spread into Iraq.