Complex sentence: The rivalry began after the end of Bosnian war, mostly because of conflict between Bosniaks and Croats during war where the city of Mostar was divided between the east side which is predominantly Bosniak, and the west side which is predominantly Croat.
Split into simpler sentences: The rivalry began after the end of the Bosnian war, mostly because of conflict between Bosniaks and Croats during war. The city of Mostar was divided between the east side which is predominantly Bosniak and the west side which is predominantly Croat.