Is it true that the Catholic Church supported the actions of the terroristic Croatian Ustashe group in the 1940s? The Ustashe apparently genocided many, including Orthodox Christians, forced conversions to the Catholic Church, exiled many, etc. I've read that many members of the Church were supportive or at least complicit in the Ustashe, even Pope Pius XII. How true are these acuusations? And has the Church in Croatia, or the Vatican ever apologized or said anything about this?
https://en.wikipedia.org/wiki/Ustashe
https://en.wikipedia.org/wiki/Jasenovac_concentration_camp
https://en.wikipedia.org/wiki/Independent_State_of_Croatia
https://en.wikipedia.org/wiki/Aloysius_Stepinac#Mass_killings_and_concentration_camps
Thank You.
I don't know about who supported the Ustaše, however, I do know for a fact that the genocide allegation is a hoax, and Jasenovac is a hoax.
Here is a book about what actually happened. This book was published in England because nobody would publish it in Croatia. Mrs Matković somehow managed to get access to some of the communist archives sealed for over 60 years. Most of the information and docuмents concerning WW2, the homeland war, missing persons, mass murders, etc. are in Belgrade and nobody is allowed access to them. Yes, even notices of whether someone's son is dead or alive and where he is buried. This is a major political issue in Croatia, or at least it used to be.
To anyone familiar with the Holoax it will come as no surprise that the nαzι-affiliated Ustaše were staunch anti-communists, however, they were even very friendly with the Church and instituted many Catholic policies. For example, anyone found with pornography was executed and periodically homes were searched for pornography, there was mandatory catechism from grades 1-12, and the like.
The claim that Cardinal Stepinac forcibly converted anyone is false. I don't know about whether somebody else was doing it. Of note is that Stepinac resisted the communist attempt to sever the Catholic Church in Croatia from Rome (like Malta) and was therefore imprisoned in a show trial and slowly poisoned to death.
Regarding Jasenovac, there's a billion ways to prove it is a hoax, but here's some mathematical proof.

As you can see, the lazy Chetniks (Serbian communist soldiers) preferred round years and years ending with 2 while inventing victims.
There's a whole study analyzing birth date distribution in Jasenovac compared to other camps published in a respectable mainstream journal:
https://ieeexplore.ieee.org/docuмent/9306761 (tvor.hr)
(P.S. this analysis takes other camps', such as Auschwitz, death tolls as fact. So it seems the Jews were at least smart enough to follow a normal distribution of birth dates while the Serbs were... let's say lazy...)
Of course, the publishers distance themselves from the facts:

The Jasenovac hoax was, of course, invented to distract from Communist murders and actual genocide that was taking place against Catholics and, funnily enough, Jews in Serbia. Yes, the non-nαzι Serbs exterminated almost all Jews in Serbia, just because they hate them.
Another "funny" fact is that during the homeland war the population of Serbia actually increased while Croatia's was severely diminished. Today, around half of Croats live outside of Croatia.
If you want to research an actual tragedy that you will probably find excused to no end on such idiotic sources as Wikipedia, you can look up the Bleiburg massacre (in Blanka Matković's book), colloquially called the Bleiburg way of the Cross, in which the English turn over around 400,000 prisoners of war to be massacred, almost half of them Croats.