I don't know which one Jesus believed and neither do you. The reason is, He never took a position on this.
Some people want to have their cake and eat it too. They want to use the words of Jesus to condemn war, even though Jesus never did. Then they want to turn around and argue for something Jesus never approved, the just war theory. You can't have it both ways. If you want to argue against war (mis)using Jesus' words, you cannot argue for a just war using Jesus.
The fact is, Jesus remained silent on the issue of how a government should handle war. He never advocated militarism, pacifism, or a just war doctrine.