Is war a necessary evil?
Yes, but is it really "evil". Gaining land for expansion. Taking resources for development. Taking Food from inferior peoples. Everyone who makes war says, "God is on MY side". Enslaving others to release the home population for better things.
You can't "coexist" with people who want to kill you.
It sometimes is indeed a necessary evil.
WWII was a perfect example of necessary evil, fighting against two fascist nations that were exterminating millions of innocents.
It's often a consequence of worldwide capitalism