The breakup of Yugoslavia involved brief conflict in Slovenia, and longer wars in Croatia, Bosnia and Herzegovina, and Kosovo. I know that Serbian paramilitary groups were involved, but the Yugoslav People's Army was also involved. Certainly involvement of aircraft and large ships seems like their involvement, and not something paramilitaries could do on their own.
It's interesting how such a large and well established army lost against Croatia, who were building up their armed forces from almost nothing. It's also interesting how JNA did seemingly very stupid things, like bombarding civilians in Dubrovnik's old town.
I'm curious what happened inside the JNA, in their chain of command. How did they become willing to attack people who they were until recently meant to protect? How did they accept the change of command, towards becoming more like a Serbian army? How did they lose so easily? Why were they willing to engage in war crimes which didn't seem useful for accomplishing military objectives? What happened to officers and soldiers who weren't willing to do these things?
I suppose some of this has been documented by the International Criminal Tribunal for the former Yugoslavia (ICTY), but I'm looking for a summary of what happened.
Yugoslavia didn't break up all at once, it took a decade. When Slovenia and Croatia republics started to separate, the rest of the country was still called Yugoslavia and JNA belonged to them. Serbia had by far the most influence in JNA even before the breakup, the chain of command was cleansed of all 'weak elements' years if not decades in advance as fears of a breakup grew.
Serbian paramilitary and JNA were on the same side, coordinated from the top, but they were still organized as separate units on the ground, with the paramilitary mostly doing the dirty work and JNA trying to keep up appearances. Eventually Serbs in Croatia and Bosnia formed their own military and JNA at least formally moved back to Yugoslavia (Serbia+Montenegro). In the end even that broke up and what was left of JNA became Serbian army.
When operation Oluja/Storm took place this was 4 years after the start of the war. JNA was no longer present. Croatia had almost 10 times as many troops as local Serbian Krajina forces, although Serbs were still well armed. But at that time Croatia has gained significant support from the west (NATO, USA in particular). Even if Croatian forces acted alone, they received support in the form of military intelligence (satellite surveillance, planning ...) for sure. At that point Serbs didn't stand a chance as they were totally uninformed, apart from being outnumbered and not so well organized. Russia didn't have the capability to make any impact in the events, help the Serbian side, so the result was quite obvious, at least when looking back. But at the time nobody had a clue this operation was coming, when it happened it was total surprise.