I found a few sites that do a good job in covering different issues concerning the theology of war. Though war is more a matter of secular concern, Believers must reconcile the issue of killing and the teachings of the Bible. Just as the shepherd must kill the wolf and the bear to protect the flock, Believers need to protect not only themselves, but those who fall under his protection — and in the face of unrelenting hostility, killing is sometimes the only option.
(PDF) A Christian Perspective on War: Theological Reflections and Ethical Implications
The Traditional Catholic View of Just War Theory and Its Application Today | The Fatima Center
The US Constitution says that Congress is prohibited from making laws establishing a religion, meaning that citizens are given freedom of religion. From this comes the notion of a separation of the church and state. However, in recent times, the suppression of Christianity by governments is on the rise. Many nations suppress and oppress Christianity — Communist China, India, and many Middle Eastern nations (including Israel). The Zelensky regime has recently been openly attacking the main Christian church in Ukraine, and even in America, Christians are under attack for their religious beliefs.
Though the theology of war mostly is concerned with wars between nation-states, the world in the End of Days is a more complex place. With the Whore of Babylon and the Beast growing in power as we move further into the End of Days, Believers will come under growing persecution. Believers will need to protect themselves and their flocks from the attacks of the Whore/Beast — literal attacks on their very lives. The question facing Believers is whether to remain in a passive defensive position, or to engage in a proactive war against the literal enemies of Jesus Christ?