The Bible speaks of an "apostasy" taking place in the endtimes. Throughout NT history, Christian states came into existence that later apostacized or backslid from the Christian standard. It becomes difficult to tell what a "just war" is, in these circumstances. Some say Russia is corrupt, others say Ukraine is corrupt, and yet others claim the West is corrupt. You have to use your Christian discernment to know what God would have you to support.
If you go back in history, you might look at the Crusades and conclude they were all bad. That's what most people do. But I don't believe that's correct. Some of the early Crusades may have been just, to some degree. But later on, clearly, the Crusades became a corrupt endeavor. Christians came and went spiritually, just as they do today. But you can't say that because some Christian wars were unjust that all Christian wars were unjust.
Israel had precisely this situation in the books of Kings and Chronicles. At times, the wars were just, and at other times they were not. None of this means that wars that were just should've been opted out of. On the contrary, God called Hebrew men to participate in just wars, as I already related to you. It's in the Bible, and Israel is an example for Christians in the NT era.