Martyn Lloyd-Jones — Welsh Clergyman born on December 20, 1899, died on March 01, 1981

David Martyn Lloyd-Jones was a Welsh Protestant minister, preacher and medical doctor who was influential in the Reformed wing of the British evangelical movement in the 20th century. For almost 30 years, he was the minister of Westminster Chapel in London. Lloyd-Jones was strongly opposed to Liberal Christianity, which had become a part of many Christian denominations; he regarded it as aberrant. He disagreed with the broad church approach and encouraged evangelical Christians to leave their existing denominations. He believed that true Christian fellowship was possible only amongst those who shared common convictions regarding the nature of the faith... (wikipedia)

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.
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.
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!