There is no such thing as a "just war" according to the New Testament. That's why most Christians who argue for a "just war" appeal to the Old Testament. But, to use the Old Testament to justify war is about as precarious as justifying polygamy (Gen. 30) or divorce (Ezra 10:3) or adultery (Gen. 38). No doubt, some might point out: but God never told them to marry another or divorce their wives or commit adultery to preserve a blood line. He did say to Joshua to "utterly kill every enemy." Ah, but therein lies the rub: God used to say "kill your enemies," but now he says "love your enemies." He used to say "an eye for an eye," but now he says, "take no revenge." At least, that's what Jesus said God says. Was Jesus right about God? Has God "changed his mind"--once he was for genocide but now he's against it? That's the question "just war" theorists must answer. Be careful little mouths what you say! You might just find yourself disagreeing with the one you claim to follow.
Jesus didn't have the same sense of justice as we do. For me, the classic example is found in Luke 12. A man asked Jesus to take up his cause. The injustice? His brother was not obeying the Torah. So, the man appealed to Jesus: "Teacher, tell my brother to divide the family inheritance with me." What Jesus should have said was, "Let's go find this scoundrel and teach him a lesson about greed." Instead, Jesus warns the victim about the dangers of "every form of greed." If Jesus were out to right every wrong, he shouldn't have walked away from this fight. He should have corrected the injustice. He is, after all, the righteous one. But, this injustice wasn't worth his time. Instead, he said (somewhat callously), "What's that got to do with me?"
Jesus wasn't out to make sure people get what they deserve. In fact, he lived and died so that none of us would get what we deserve. In a "just" world, everyone is supposed to get what they deserve. We make laws and wars to make sure of it. We know that law and order doesn't work sometimes. We know that war is bad. Innocent people can suffer from both. But, we believe the lesser of two evils is required if we're going to strive for justice. And yet, most of the time, preserving our justice comes at the expense of another. Justifying war seems just as easy as justifying our sin.
So, how did Jesus deal with this mess? What did he do? Jesus believed the only way we can do justice is to show mercy. That's what the cross proves to us. To quote Paul, "while we were yet sinners, Christ died for us." And, here's the hard part: Jesus expected us to follow his lead. Paul did. Peter did. John did.
Honestly, I'd rather have "justice" on my terms than his.