Sorry to be so brief with these answers (and to ignore some outright), there's A LOT of questions to tackle and I'd end up dumping a huge essay in here otherwise. I'm sticking to the Ustasha if that's okay, as I've written a lot about them in the past.

1: Why did the Ustase form? What were their goals?

At the beginning of the 20th Century fascist movements were seen - in the eyes of their supporters - as a revolutionary, 'energising' creeds capable of transforming moribund modern states built on compromise into bold new industrial titans. The Ustasha Croatian Revolutionary Organisation formed in 1930 out of assorted student organisations, far right groups and the militant wing of the Croatian Party of the Right (HSP) with a view to creating an independent Croatian state separate from the new Kingdom of Yugoslavia, with its Serbian monarch and its Serb-dominated administration. This 'Greater Croatia' would include Bosnia and Herzegovina, and be purged by force of all undesirable "aliens" - specifically Serbs. Much like Italian Fascism and the British Union of Fascists, anti-Semitism wasn't originally a characteristic of the Ustasha - indeed there were right wing Jewish students among its early ranks - but this changed as the ground entered the orbit of Germany's National Socialists.

It seems very modern, but with the blessing of Mussolini's Italy the Ustasha leadership fled Yugoslavia and set up - for lack of a better word - terrorist training camps, attended not just by radicalised young Croats from Yugoslavia but members of the larger emigre population in Central Europe. These camps were run on strict military lines and recruits swore blood oaths to the leadership, including future 'Poglavnik' Ante Pavelić, and the graduates of this school undertook campaigns of bombings and assassinations in an attempt to destabilise Yugoslavia.

2: How much of Croats supported the Ustase? How much were Partisan or Peasant Party? Some say that Croats were ecstatic when the Usatse took over others say opposite.

The Independent State of Croatia formerly sprang up in April 1941 in the wake of the Axis advance and the reception from Croats was largely popular. Writers within Croatia compared it to the euphoria of Croatia being detached from Hungary in 1918 and a US diplomat reported on cheering crowds greeting the German army as it rolled through Zagreb. By the end of 1941, the Ustasha gained approximately 150,000 new members. As for the Croatian Peasant Party, in August 1941 a right wing faction of the party actually entered the Ustasha government, with regional branches of the party swearing allegiance to the Ustasha regime. The Croatian Union of Workers followed suit.

I can't answer comprehensively about the exact nature of the support, I would suspect (from an educated perspective, as much as one is allowed to suspect anything in AskHistorians) enthusiasm for an 'independent' Croatia bled into enthusiasm for the Ustasha. It's worth remembering that during the Habsburg era, many Croatians were fiercely loyal to the crown but dismissive of Hungary and the Hungarians they were officially attached to, and would petition the monarchy to intercede on their behalf. To that end there's definitely a rose-tinted nostalgia toward the Germanic world as the guarantor of Croatian interests and dignity as it was easy to see "noble Austria" as the distant father who doesn't know that the older brother - Hungary - is beating all the younger siblings.

Not all Croats supported the Ustasha, many remained invested in the Yugoslav state and some of them supported the Partisans. But alas, I don't have numbers to hand. There's reports from the Wehrmacht of the Ustasha's murderous excesses "horrifying many conscious Croats" in the summer of 1941 and there were sporadic attempts to curb some of the bloodlust across the rest of the war, which suggests that Ustasha popularity hinged largely on Croatian independence and not ideology.

3:Why did Nazis support Usatse and why did Ustase like the Nazis considering Nazis hated Slavs?

By the time Italy and Germany rolled into Yugoslavia, the Axis war machine was becoming stretched. Troops were needed for the impending Operation Barbarossa and so local collaborators were being increasingly relied upon to garrison and maintain order behind the frontline. The Ustasha were Slavs, but thanks to Croatia's legacy as a frontier of the Habsburg Empire, they were sufficiently Germanised culturally to be passable - the Ustasha themselves contrived an Aryan linage to smooth things over, insisting that Croats were in fact the descendants of Vlachian people, positioning them neatly as the antiquity's last line of defence between Central Europe and the "Asiatic" South Slavs and Turks.

4: Is it fair to say Croatia was a united Nazi state like Germany, as in is there a parallel? Or were Ustase seen more as a terrorist organization?

Ustasha ideology had been woefully vague on exactly the brand of government they were attempting to create and how it would be run, and the lack of ideological focus attracted a wide variety of adherents on the far right - from deeply Catholic clerical fascists seeking to overturn the secular new world or the Orthodox overlords to metropolitan romantic "Glory to the soul of the flaxen haired peasant" types seeking to overturn the homogenous new Yugoslavia to working class atheist anti-Semites in the National Socialist mould seeking to overturn the 'worldwide Jewish conspiracy'. The biggest division, meanwhile, was between the Ustasha who had lived in Yugoslavia throughout the organisation's exile and the Ustasha who had trained in the camps, the latter were far more confrontational in their politics and in their vision for an independent Croatia than their sedentary colleagues.

The acceleration of their programme from the political fringes to statehood resulted in a sort of feudal interpretation of government where these differences in ideological background became apparent and the further you got from Zagreb, the more it felt like Croatia was being governed by fascist warlords arbitrarily following and ignoring directions from the top. To refer back to Answer 2, the inclusion of the right-wing of the Croatian Peasant Party etc alarmed the Ustasha old guard with what they saw as the dilution of their purpose, but it does suggest that the Ustasha was seen as a single issue party that could be adopted by existing political factions.

5: How brutal were Ante Pavlic's Croatia? I couldn't find much info on the ladder. How many victims, perpetrators.

The Independent State of Croatia was genocidal to a degree that surprised even the Germans. While their policies for government were ill-considered, their plans for Croatia's 'foreign' population had been gestating throughout exile. The primary target of their ire was of course Orthodox Christians (Serbs), with Gypsies and Jews coming second, and to this end they established the largest concentration camp outside of German administration - Jasenovac. By the time Jasenovac was liberated at least 100,000 men, women and children had died from malnutrition, starvation or physical abuse.

Violence against the Serbs was referred to as a "revolution in blood" that would purify the nation and the Ustasha implemented a 'third' rule - kill a third, convert a third to Catholicism and drive out a third. Serbian and Jewish businesses were nationalised, Serbs and Jews were barred from public sector jobs and in some areas from public spaces such as parks and theatres. Much as they had in Nazi occupied zones, ghettoes sprung up and genocide followed in its wake.

In the countryside Ustasha militias swept through towns and villages in an orgy of violence aimed at the Serbs - men, women and children were stabbed or bludgeoned to death, shot and thrown in pits, or locked in their churches and set alight. The conservative estimate is that out of Croatia's pre-war population of 1.9 million Serbs, between 200,000 and 500,000 were killed by the Ustasha, either by death squads or in the camps.

On a day-to-day level, the Ustasha created a forerunner to the surveillance state. Newly formed propaganda ministries censored all press, while curfew turned the streets of Zagreb into a ghost town - posters blared "Nothing escapes the eyes of the Ustasha!"

Sources: The Ustasha Regime and the Cultural Politics of Fascism, 1941–1945 by Rory Yoemans, In the Shadow of Hitler: Personalities of the Right in Central and Eastern Europe by Rebecca Haynes and Martyn Rady, and Croatia: A Thousand Year Dream by Marcus Tanner

1. Both were founded as nationalist organizations seeking to establish an ethnically homogonous Croatia/ Serbia. The Ustase were fascist, the Chetniks monarchist. Nationalist tensions was already high during the existence of the Kingdom of Yugoslavia, with fierce dispute and conflict and a basically paralized government due to this nationalist infighting.

2. The Nazis needed a somewhat legitimized ruler in the Balkan, as they didn't have the ressources to spare for fighting Partisans and occupieing the country. They needed the ressources for Operation Barbarossa against the Soviet Union.

Nazis often acted pretty pragmatic when it came to ideology. The may hated them, but the needed to work together with them for now. They also created a slavic Puppet state in Slovakia. What would've happened to fascist Croatia if the Nazis won the war is somewhat guessing work.

5. I don't really know about Nedic, but Ustasha Croatia was very brutal. Jasenovac was a Concentration Camp similar to Nazi ones. About 100.000 people were murdered there alone. The total people murdered is somewhat unclear, also because it today is very much a nationalist fight, and during Yugoslavian times there was more or less silence about it.

6. This is somewhat a complicated question. Because the first question is, how similar were the Chetnik and Ustashe organizations founded again in the 90ies to their forefathers during World War 2?

I'd say that they very much bear responsibilty for what happened in the 90ies, especially because War criminals were for a long time, and sometimes even today, protected by them. Also because they are basically the same state tradition.

I'd also argue that todays Croatia doesn't have a legal connection to the mass murder during World War 2, but has somewhat of a moral obligation to not forget it and keep the memory of its cause and its victims alive. But this is more or less wishful thinking.

Basically the same holds true for Serbia.

9. As far as I know, there has been done very little in this direction. (Correct me if I'm wrong!) Also due to the circumstance that any attempt at truth commissions haven't worked out because of various reasons.

Most of the questions have been directly addressed, so I'll add a more nuanced perspective on some issues. Like the previous poster, I specialize more in the Croatian case. Full disclosure: I am a Serbo-Croat speaker, and can recommend secondary literature in both English and native languages.

Regarding the WW2 period, a distinction must be made between the Ustasha and Nedic groups. The former had the full support of Nazi Germany as well as popular support from the Croat population. The latter was a Belgrade-based Nazi puppet regime which was not supported by the General serbian population. The difference is that if German support was withdrawn, the independent state of croatia was quite capable of functioning as a legitimate state in the eyes of its populace. The Nedic regime would have been toppled immediately. The anti-Nazi Partizans were a majority-Serbian force, but large numbers of Croats and non-Serbs were members, even at the highest command levels. The majority of the Croatian population however supported the Ustasha regime under Ante Pavelic.

Now, the Chetniks can't be associated with the Nedic government. They were royalists, and thus anti-Nazi by definition. However, collaboration did occur and the facts on the ground never reflected official ideology.

How brutal were the Ustasha and Nedic governments? Very. The Ustashas however were exponentially worse, in every quantifiable way. The Jasenovac concentration camp was the fourth largest in Europe, and the only one run by non-Germans. As another poster mentioned, exact figures for Serbs, Jews, Roma, and anti-fascist Croats killed there and in Croatia in general are difficult to ascertain. Tito-era blanket repression, modern propaganda, and large scale population movements during and after WW2 make it difficult to compare census data. Modern Croatia does not commit any significant resources to answering the question. The most probable answer is that hundreds of thousands of Serbs and others were killed by the Ustasha regime. A good analysis of mass killings is: Tomislav Dulic, Ethnic Violence in Occupied Yugoslavia: Mass Killing from Above and Below, in Dejan Djokic's New Perspectives on Yugoslavia.

The Nedic regime was brutal, but fundamentally different from the Ustashas. It did not run concentration camps anywhere near the scale of Jasenovac, it had no large internal non-Serb population to repress and murder, and it was focused mostly on enforcing Nazi policy in Serbia. As it did not enjoy popular support, considerable resources went to keeping Chetnik and Partizan opposition in check.

Why was Ante Pavelic so anti-Serbian? This is a difficult question, and though I can expand on it enormously, it's a topic for a separate thread if you wish to make it. I'll keep it short. Croatia has had a complicated and often antagonistic relationship with its [large] non-catholic minority for half a millennium, but it was never violent in the large-scale sense until the interwar period. The Serbs and Croats had differing conceptions of what the kingdom of yugoslavia was supposed to be, owing to differing historical experiences and an uneven power equilibrium which rendered Serbs more politically and militarily powerful than Croats. This led to interwar political deadlocks, vitriolic nationalism, and odd versions of ethno-nationalist fascism springing up. Croatia was a country which defined itself by a language it shared with serbia, but a catholic faith which was antithetical to the Serbs' orthodox faith. But how did this definition mesh with the reality of a quarter of its population identifying as Orthodox and Serbian? Odd contradictions abound. The 19th century 'father of the croatian state', Ante Starcevic, was half-Serbian. Franjo Tudjman, the nationalist president of Croatia in the 1990s, has a daughter who married a Serb.

I'm on my phone, so I apologize for the poor formatting. My response is admittedly all over the map, so I'll leave off with a few good sources to get you started. The previously mentioned source by Glenny is also good, though he's not an academic but a journalist so I would be careful.

Dejan Jovic, Yugoslavia: The State that Withered Away

Dejan Djokic, New Perspectives on Yugoslavia