It's very complicate and difficult question but answer is simple.
We, people, separated from God in the Garden of Eden.
From that moment, people are in constant conflict and war and there is no peace any more.
Wars and conflicts are not only on the world level.
Just look what is happening in today's families, society ...
Since we are people who do not have harmony in ourselves but conflict, our families, societies, nations and world are also in conflict.
It's inevitable.
Only solution to end this conflict is to come back to God, our creator.