Here's a reframing of an issue that's been discussed a good bit these last few years.
It occurred to me that one way to look at war and peace and the Bible is that the norm in the Old Testament and New Testament was peace. God expected God's people to be peace-loving workers of justice. Period.
That norm was interrupted at times (seemingly quite often in the OT) when God specifically told Israel to engage in a specific type of war (not depending upon her army, but upon God, and often involving wiping out every man, woman, child and puppy dog in the process).
Then we received a definitive teaching from Jesus, telling us that we ought to love our neighbors, friends, enemies and fiends.
For me, this is the last word from God. I'm willing to suggest that God may indeed want to call me to go to war again, but it'll have to be pretty specifically from God, not a questionable ruler. In the meantime, the norm at least for Christians is to follow the last word we have from Jesus.
Love your enemies.