There was a time when protestants and catholics killed each other.
There was a time when english and french killed each other.
There was a time when french and germans killed each other.
There was a time when bosnian, serbs and croats killed each other.
Now they don't.
Peace is possible. Peaceful resolution of even the most intractable conflicts is possible. No matter how complex the territorial, religious, ethnic, cultural differences... there is a way to live together in peace. It has been done before. Let's not just learn from the mistakes of the past, but also from the success of the past.