All serbias fault
WW1:
Serbia did assassinate arch duke because they wanted Austria-Hungary out of their land. Russia being the warmonger they are saw some catholics beating a poor fellow orthodox country, so they declared war. France the UK being allied to Russia decided to join in. Germany saw Austria-Hungary getting beat up, so they decided to help their fellow Germans out. Also Ottoman joined in too. They lost and Yugoslavia was formed which incorporated Serbia.
WW2:
So Yugoslavia gets its ass kicked by Germany and the Croatians are put in charge, this lead the serbs to take up arms against the German while croats others in the Balkans decided to take up arms under the communist banner. Both wanted to rebuild Yugoslavia, One group fought for the axis, another for serbs and sometimes worked with the germans to beat the commies, and the commies which one in the under glorius tito.
Cold War-Present: Tito gave USSR the middle finger and remained neutral despite being a socialist nation. They often played both NATO and the Warsaw pact for favors and stuff. However Tito was the glue that kept the nation together, with his death the whole thing fell apart. The Serbs once again decide that they want build a Serb dominated state and decide to go kill everyone, again leading to one of the biggest conflicts in Europe since WW2.
So yes, all
Serbias fault.