I’m not a professional historian but I’m from there so hopefully I can explain. Also keep in mind that most Croatian hated the ustatse but the Croatian generals and clergy sided with the ustatse. When franz Ferdinand was assasinated croatians were pissed. Franz ferdinand wanted to make croatia a constituent with Austria and Hungary because they had fought most of their wars for centuries. Serbia killed franz on purpose because of something called “greater Serbia” geneticists and historians have proved it is bs but it’s still a popular thing in Serbia. Essentially serbs believe that all bosnians and hercegovs and Dalmatians are actually serbs but they go under fake identities(bs) and franz Ferdinand would’ve made croats loyal to the crown by giving them a bunch of power. Croats were actually the ones that originally proposed yugoslavia and although mad at serbs were still willing to form it after ww1. Now here’s where it gets political. Yugoslavia was mostly controlled by the Serbs after ww1 and again serbs really only cared about greater Serbia not Yugoslavia. They tried to “convert” Croats into serbs and also suppressed Croatian media but most of all the exploited the Croatian economy, after all at the time Croatia had the most industry, and the Serbs wanted to move the factories to help their local economy. Now stjepan radic who was a political genius called the serbs out and said “hey this is bs, we want to have a more autonomous states within yugo” now stjepan believed in democracy but he still believed in a unified serbo Croatian country, just not one were serbs were bullying croats. Because of this stjepan radic was shot and killed in the middle of parliament. Now croats were pissed, and when some people are pissed they turn to extremism unfortunately. Although most croats now is wanted an independent democratic Croatia, the clergy and military in Croatia saw an opportunity. When ww2 came around, most croats fought to defend their land, but once it was taken the nazis put the ustatse in charge. The ustatse were one of the most terrible groups of people ever. The killed 170,000 serbs whether they were babies children women whatever, they killed them. They also helped the nazis kill another 180,000 or so serbs, so in total roughly 330,000 serbs died on ustatse soil, and the ustatse killed 170,00 of those. Now keep in mind again most croats were either CPP supporters(Croatian peasants party aka stjepan radics party) and there were also some communists. Croatians ended up defeating the ustatse and some of the chetniks(Serbian nazis that still exist today) but the chetniks managed to kill 70,000 croats and 5,000 bosnians in ww2. This leads way for the exact same thing to happen where Serbia controlled commie yugo and exploited the croat economy. Also this time around Serbia tryed to “learn from their mistakes” so all the police officers were serbs, most of the teachers were serbs, all of the government besides Tito and a couple exceptions were serbs and all the industry was built in Serb towns, on top of heavy secret police activity in Croatia. As we all know this lead to the 90’s wars and that’s when the serbs commited mass genocide against croats and bosnians. Essentially this story is that the Serbian government wanted to create “greater Serbia” and killed anyone in their path. That led to the powerful people in Croatia becoming ustatse which led to more genocide on both sides and then led to oppression and eventually even more genocide. Although f the ustatse and I hope they all rot in hell, you can’t fuck with a group of people and not expect them to respond.