Did Serb nationalism play an outsized role in the breakup of Yugoslavia?

Restricting this answer to the objective, "What reforms the Milosevic introduce?" From the Britanica article:

As Serbia’s party leader, Milošević demanded that the federal government restore full control to Serbia over the autonomous provinces of Vojvodina and Kosovo. And at a time when the federal government was trying to introduce free-market reforms in order to relieve the faltering Yugoslav economy, he emerged as a leading defender of the socialist tradition of state economic intervention, attacking economic reform for its social costs.

In 1988 Milošević replaced the party leadership in Vojvodina and Kosovo provinces with his own supporters, and in 1989 the Serbian assembly ousted Stambolić from the republic’s presidency, replacing him with Milošević.

In 1990 Milošević pushed through changes to the Serbian constitution that curtailed the provinces’ autonomy.

This single action is probably enough to justify breaking up the country; "curtailed autonomy" is always something imposed, never celebrated.

He resisted a growing movement in favour of multiparty elections, and he sought to use the extensive Serbian diaspora throughout Yugoslavia in his fight against confederalism, a looser union of sovereign republics that was advocated by the leaders of Croatia and Slovenia.

Once again, history is full of examples of "the fight against confederalism..." being the justification for the breakup of a country. Once you start fighting against confederalism, you're pretty much inviting revolution. And in fact the next year, Croatia and Slovenia elected officials who declared independence.

TLDR:

Prior to economic troubles provided by economic mismanagement courtesy of the IMF, most Yugoslavians were not nationalistic separatists; they wanted a unified Yugoslavia. After economic troubles, IMF/Nato in its cold war methodology fanned the flames of nationalism to put nationalist into power. In this view, Milosevic was a tool of the greater powers that had the support of people who were "left out in the cold" by IMF economic restructuring.

Long answer:

There are a few thoughts about Yugoslavia and the break-up that get bounced around. And then, there is what really happened. I will try to first enunciate these meme-style views, then present what I feel is a "centrist" view.

The Conservative Western Press View

Yugoslavia was created artificially, bringing together 3 natural enemies: Catholics, Muslims and Orthodox Christians. It was kept together by Tito who was a strong Stalin-like communist dictator. Once Tito died, there was no strong hand to keep the various ethnicities in line. The Slovenians and the Croats wanted independence and to have free democracies with free markets. The Serbs, however, wanted to stay communist and have a Serb dominated Yugoslavia, after having been held down by Tito. So when Slovenia and Croatia seceded, they attacked them and tried to ethnically cleanse them in order to have more territory. This became even worse when Bosnia wanted to be independent, because the Bosnians are Muslims and are even more defenseless and hated by the Ultra-Nationalistic Serbs. So the EC, then the UN and finally the US had to step in to stop Serb aggression, which they did by sanctions and bombing which eventually stopped the ethnic cleansing and Serb aggression. Then they did the same thing in Kosovo before the Serbs could kill all the Albanians. So it was a great moment, because it showed that the EC, Nato and the US could, with UN backing, do something positive to halt the bad guys in the world from doing evil Hitler genocide stuff in the post cold war climate. This is called Responsibility to Protect (R2P), which involves humanitarian military action based not on the attacking countries' interests, but rather on enforcing international standards of human rights and justice.

A reference related to this view point can be found in A Problem from Hell: America and the Age of Genocid. I want to point out that this is the narrative I recall from the press in the 1990s. In retrospect, it seems a little bit too clean. We all know that the United States is not so altruistic; certainly there were some ulterior motives, right?

A Pseudo-Marxist View

The Second Yugoslavia was founded by the anti-fascist resistance led by Tito, which defeated the Nazi collaborator Croat Ustashis who were responsible for the a thorough genocide. This was part of a larger trend, where the anti-fascist resistance in countries like Italy, Greece, Yugoslavia, USSR were all intimately linked with the communist parties. As soon as the fascists were defeated, both the USSR and Nato moved in quickly to "re-establish order." In the case of the USSR, this meant solidifying Stalin's control over the parties, and in the case of Nato, it meant simply wiping out the partisans and putting the fascists back into power (see Operation Gladio). The two powers thus reached an agreement at Yalta on how to divide Europe between them, with Nato getting Greece and Yugoslavia being split between the two.

So Yugoslavia had a mixed character: it was part of the eastern-bloc Mutual Assistance Council Thinger, but was not in the Warsaw pact. It received military aid from Nato, but had a soviet-style economy with a one-party system and worker protection: pensions, housing, education, health. Since it was on the border, trying to live in both worlds, this was an opening for Washington to undermine the system using the IMF (International Monetary Fund). They needed loans and the IMF would lend them money, but with the usual demands for structural "reforms" meaning cuts in social services and disregard for unemployment. This led to the usual IMF destruction: the reforms squash growth, and the shrinking economy means you need more loans. Cuts in social services and unemployment led people to turn towards various forms of extremism, including nationalism.[1]

In this mix, it was Slovenia that was richest and most connected economically to western suppliers and clients, so Slovenia was intentionally pushed to secede: "why should you have to pay all these taxes that just go to less developed republics like Macedonia and Serbia? Plus you have to deal with all the unemployed immigrants that come from there. Plus on a deeper more racist level: you’re Europeans like us, not like those backwards communist Serbs."

The Slovenian and Croatian intelligentsia turned this into the anti-communist rhetoric. "We want to be advanced and free and democratic; they want to be backward and communist." So using this opening, Slovenia was a great opportunity for the West to get into the Yugoslav markets from which it had been locked out. The EC, and especially Germany, Austria, and Hungary, peeled away one by one at the republics: first Slovenia and then Croatia, then Bosnia, and finally Macedonia were opened up.

But the ones that wouldn't open their markets, that wanted to hang on to their decent social standards, that did not want to privatize everything, were the Serbs/Montenegrins. So they had to be punished. And if you look at every action that was taken, there was no type of standing up for principle or defending human rights; the only constant in the decisions was, you have to punish the Serbs. So for example: the Croatians have a right to secede, but the Bosnian Serbs cannot secede from Bosnia. you have to condemn genocide in Srebrenica, but not in Mostar. Etc.

A source for this viewpoint is To Kill a Nation: The Attack on Yugoslavia. This book was written by Micheal Parenti I want to point out that while this account is vastly different from the one we encountered in the US press in the 1990s, it is a viewpoint that is held by some people. Whom? I am not sure, but a quick google search shows that at least some people find it plausible. Here's one link from that search, How the IMF Dismantled Yugoslavia .

Another Source

Let's look a a book put out in 1995 by The Brookings Institute, which is a policy research institute that is labeled as "centrist" in the spectrum of US politics; so it might be considered "conservative" in comparison to other countries. The book is called Balkan Tragedy: Chaos and Dissolution after the Cold War and was written by Susan L. Woodward. Looking at the list of what she has written, I think somebody from SE should email her and ask her to answer this question for us.

In this book, Woodward firmly rejects "The Conservative Western Press View" as not fitting at all with the way things actually happened, and she spends about two thirds of the book showing how wrong the paradigm is, with excellent original source material from mainstream sources.

She parallels Parenti's work to a large degree, but with a few interesting differences:

• for Parenti, there is a conscious concerted effort to break up Yugoslavia and punish the Serbs; for Woodward, it's a tragedy that happened due to Western diplomats being stuck in a cold-war mindset and seeing the ethnic divisions in the country instead of the actual economic and social situation. In other words, Woodward says the problem is diplomats believe the "The Conservative Western Press View", vs. Parenti who says "no it's all part of a systematic plan."
• Woodward assumes: well it’s a good thing to institute free-market reforms and liberal democracy and all that crap, but it was done in a brutal, undemocratic way in Yugoslavia that ended up destroying the country. According to Parenti, that IS the problem: wherever there is a decent system for workers, it will be attacked by market austerity, so Yugo should have just been left alone.
• Parenti sees Kosovo as just another attack on Serbia and part of the same assault on a decent social system. Woodward, while being against bombing Serbia and against sanctions as counter-productive, nevertheless feels the Albanian majority in Kosovo was being abused by the Serbs and so should have been allowed sovereignty.
• Milosevic: Parenti sees all the western attacks on him, but sees no evidence for him to be called a "monster;" Woodward sees Milosevic as an extremist nationalist that Nato kept in power, but who did not represent the majority opinion in Yugo/Serbia.

Woodward's contentions in brief:

Use of extremism to destroy a nation. She does a damn thorough job showing that prior to the 90's, most Yugoslavians were not nationalistic separatists, but EC/Nato diplomacy did backflips to ensure that minority-supported nationalist leaders were put into power; and that they became the representatives for entire republics (not just their respective ethnic groups). This argument can go a long way to explaining a lot of stuff: Nazi Germany destroying Wiemar, Young Turks destroying Ottoman, etc...

Use of austerity to destroy a nation. Same point as Parenti, just without the assurance of a pro-socialism perspective. But she spends about the 1st half of the book on the sordid history of IMF indebtedness.

Inability to define crisis as civil war/war of aggression/state-building. The Germans (and subsequent diplomatic followers) doing intellectual gymnastics to try to define the situation: The Slovenes and Croatians want to secede? Must defend their right to sovereignty! The Bosnian Serbs want to secede? No way, sovereignty's not for everyone! So they go through these series of contradictions in order to continuously re-define the situation to suit their true aims. Here Woodward just chalks this up to confusion, an inability to understand the complex situation, being stuck in a coldwar mindset, etc., Parenti would just say it's all BS because all they really want to do is to punish the somewhat socialist Serbs.

Inability (unwillingness) to reach comprehensive solution before jumping to secession

Nations or states? Muslims or Bosnians/Bosniaks Woodward makes a great point about Ali Izetebegovic saying that his position was completely misunderstood (on purpose?) by the EC/Nato. He was the rotating head of the 7-member Bosnian presidency at the time of secession, and was also head of the largest Muslim nationalist party in Bosnia. They conflated the two roles, essentially shutting out not only all the Bosnian Croats and Serbs but also most of the Muslims and anyone else who did not agree with the separatist project (ie, most Bosnians). In order to do this, they had to shuffle terms around like Bosnian/Bosniak.

How to manipulate the Nato into bombing. Woodward does an excellent job showing how the Serbs/Monts were light years behind the Slovenes, Croats and Muslims in being able to manipulate western opinion to force a "humanitarian intervention".

So that's what I've got, a summary of three viewpoints. I think most people will look at their own political viewpoints, and choose one of these perspectives that matches their view points. That's the trick with recent history, right?

TLDR:

Prior to economic troubles provided by economic mismanagement courtesy of the IMF, most Yugoslavians were not nationalistic separatists; they wanted a unified Yugoslavia. After economic troubles, IMF/Nato in its cold war methodology fanned the flames of nationalism to put nationalist into power. In this view, Milosevic was a tool of the greater powers that had the support of people who were "left out in the cold" by IMF economic restructuring.

Note

[1] Aka, Trump? Brexit? In this view, these are similar social phenomena.

This depends on how you define "responsible"

The immediate reason was that the Croats and Slovenes (and later all the other non-Serbian nationals) overwhelmingly wanted independence. Under those circumstances an eventual breakup was unavoidable.

Of course, you can look deeper and ask why did they want independence? Was is because of the Serbian dominance in the united state? The disappointment with the pan-national ideals of Yugoslavia? Economic considerations? The deep cultural and historical differences between the nations? Maybe local power-hungry politicians convinced them? Possibly it was part of a larger worldwide wave of nationalism (e.g. the breakup of USSR & Czechoslovakia)?

The answer, like in many similar historical cases, is that nobody knows. Possibly all the factors above (and many others) contributed in one way or another. Which was the most important? It's anyone's guess.