I’m trying to really condense this, but Balkan politics are crazy complicated. If this really interests you beyond reddit, check our books by Mark Biondich or Sabrina Ramet.

When Yugoslavia was established it was supposed to have a relatively balanced government of Serbs, Croats, and Slovenes. Instead, England and France installed Serbia’s Karađorđevic family as monarchs. The Serbian rulers were oppressive and those who weren’t Serbian or of the Serbian Orthodox faith were at best second-class citizens, and were often harassed by government-sponsored gangs/militias that roamed the countryside.

Ultimately Stjepan Radic and Vladko Macek mobilized the Croatian Peasant Party into a massive non-violent political movement and forced the Serbian government to recognize Croatia as an autonomous state within Yugoslavia in 1939.

This upset a Croatian named Ante Pavelic who led a militant Croatian fascist group called Ustaše, that was based out of Italy. Ustaše wanted Croatia to be a fully independent country, and advocated terrorism and war as the tools to get there.

In 1941 it became clear that Germany was going to move eastward and overwhelm Croatia. The Germans approached Macek and asked him to lead an independent Croatia under Nazi protection, but Macek refused (he spent the next 4 years alternating between a concentration camp and house arrest for this). The Germans soon led their troops into Yugoslavia without a clear plan to govern it.

Pavelic brought Ustaše back to Croatia from Italy and brokered a deal with The Nazis. He was installed as the leader of the Independent State of Croatia, a puppet state under Nazi authority that controlled all of Yugoslavia, and immediately launched a campaign of terror against Serbs, Jews, and Roma within Yugoslavia. The numbers are shaky, but it looks like between 45,000-60,000 Serbs were killed at Croatia’s Jasenovac concentration camp alone.