What is Just War?

It's a doctorine of military ethics of Roman philosophical and Catholic orgin studied by moral theologians, ethicists and international policy makers.

The doctrine of a just war was developed during the 2nd half of the first millennium; it sought to establish reasons why a nation might be justified in going to war. The Catechism

Justification is a difficult thing to prove, depending on who you ask. From the Greek point of view, especially that of Menelaus, the war was justified because he was chosen to be