The whole war begun when a small anti-imperial nationalistic political movement (Black Hand) challenged the Austro-Hungarian imperial status quo in Bosnia - the only remaining colonized part of the Balkans after the Ottoman Empire had been forcefully evicted from her remaining imperial territories in Europe.
Then three Empires went to war in eastern Europe, and the battles of this war were fought out almost entirely in territories where the civilian population was not from the dominant nationality of the respective empires - in the colonized areas of the empire. The battles raged on in the lands where the majority of the civilian population was ethnically and religiously mixed and as a rule mostly Polish, Lithuanian, Latvian, Estonian, east European Jewish or Armenian - instead of German or Russian.
Then the imperial control in the borderlands begun to gradually and rapidly crumble, and the imperial states initially replied with increased repression and violence, especially in the cases of Russia and Ottoman Empire. And as the state lost control to the monopoly of violence and old authorities crumbled, new political movements and "violent entrepreneurs" sprung up all over eastern Europe, further increasing the cycle of violence and destabilizing of the old status quo. The following social disaster was accompanied by massive refugee movements, Balkanization of former imperial territories, ultimately ending with the process of building new states out of the ruins of the old empires - this process was very much still unfinished in many parts of eastern Europe when WW2 begun.