It's necessary for you when the other side has decided to engage in it against you. The fact that it was technically "unnecessary" for them to start the war in the first place is rather irrelevant when they're marching down your street. The decision about whether there's going to be a war, necessary or not, has been made for you.

If you don't believe me, I'm sure there's a Kurd somewhere who would be amused to hear your theories about why their fight against ISIS is "unnecessary".