They might wage war on the basis of religion. We do not.
Christians should never wage a religious war. We should learn from the crusades that religiously motivated wars are not the answer.
As Christians we should step back from conflict and always (from a religious perspective) be opposed to war and resist war as long as possible and always resist the temptation to wade into war for religious reasons. Our weapons should never be military, but spiritual.
However, what the civil authorities do is something else.
The civilized nations of the world should begin equipping themselves for whatever it takes to overcome the ISIS threat. They should do so purely from a military and political standpoint. From our side religion should be taken out of the equation. This should enable us to make alliances with countries with Muslim majorities who also wish to extirpate the foe.
We should consider the ISIS threat the same way we considered the Nazi threat. Nazism may have been driven by a pseudo religious ideology of racial superiority, but we didn’t go in with an equally absurd ideology. We confronted the evil because it was evil. It was destroying innocent lives. It threatened our own way of life. We saw the evil for what it was, didn’t bother debating the stupid ideologies and simply rolled up our sleeves, put up our dukes and went into battle.
We should confront the pseudo religious, barbaric ideology if jihadism in the same way. Let’s not get caught up in religious arguments, racial arguments and ethnic arguments. Let’s just see the demonic, cruel and barbaric evil for what it is, get ready to fight it and not hold back until it is utterly destroyed.