Martyn Lloyd-Jones quotes:

The Bible does not isolate war, as if it were something separate and unique and quite apart, as we tend to do in our thinking. It is but one of the manifestations of sin, one of the consequences of sin.

God permits war in order that men may bear the consequences of their sins as punishment. How clearly this is shown time and time again in the story of the children of Israel!

This actual question of 'Why does God allow war?' is not considered or raised as such in the Bible at all.

Every painful consequence of sin is a part of the punishment meted out for sin.