Is it a sin to pray for peace?

No its not a sin. We cannot gain peace. We as a species will fight aginst eachother. It is darkest before dawn as they say. God has destoried cities, he even destoried the entire planet except the few who he viewed as virtueous. When the end comes it will be by death to most of our race as he has done before. Peace is a goal that we strive to reach but is always just out of our reach.