I am puzzled because some Muslims belong to the same sect but fight others based on ethnic grounds!! Look at Turks and Kurds. Look at conflicts in Pakistan and Sudan and Afghanistan.
Br Sidqi has an interesting comment. The Christians are 600 years ahead of us (and the Jews even more). I think over time they have matured and realised that they must concentrate on what unites them rather than what divides them. They have had their fair share of wars based on ethnicity. They came to realise that unless there is rule of law, there can be no real peace in the society.