How were the diverse, multi-ethnic peoples of the balkan region once held together?

The Balkan region is a very diverse ethnic and linguistic region. Former Yugoslavia was the attempt to create permanent peace in this region, especially after WW II, through so-called "brotherhood and unity" among the nations. Yugoslavia included many ethnic and religious groups, which were held together by a strong communist government, with its charismatic leader Josip Broz Tito. But in 1990-ties the forgotten conflicts between the different nations escalated ( Serbs and Croats, Serbs and Muslims, Croats and Muslims, the conflict in Kosovo ).
The diverse, multi - ethnic peoples of the Balkan region once held together through the Republic of Yugoslavia and the idea of "brotherhood and unity". 

The answer is the magnetosphere.