Christian theologians and philosophers, such as Thomas Aquinas,
Francisco Suarez and Hugo Grotius, built up a theory of the Just War:
that is to say, of the circumstances in which it was morally justified
to go to war, and of the moral restrictions on the waging of war once
begun. This theory lies behind many of the international principles
and conventions which regulate the waging of war at the present time.
A partly parallel theory was worked out by Muslim theologians.

Quite different from the theory of the just war was the theory of the
holy war.