This actual question of ‘Why does God allow war?’ is not considered or raised as such in the Bible at all. – Martyn Lloyd-Jones
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. – Martyn Lloyd-Jones
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! – Martyn Lloyd-Jones