"Yugoslavia: Death of a Nation" - Silber.

I highly recommend you read this book. The entire story is convoluted like you can hardly imagine, and this book really helps pull you through it.

The former Yugoslavia was a nation built post WWII by largely partisan/anti-fascist forces who set up a socialist state that comprised 6 different political areas. 6 republics (Slovenia, Croatia, Serbia, Bosnia, Montenegro, Macedonia) + 2 acknowledged autonomous regions (Kosovo, Vojvodina). Pre-WW1 this area was part of Austro-Hungary so suffice it to say there had been a long stretch of a higher authority clamping down on the smaller regional squabbling.

Under the dictator Tito, post-WWII, the above zones were kept largely in line, and ethno-nationalist sentiment was heavily suppressed.

When Tito died the seperate republics began jokeying for their own benefit. Further, nationalist movements began to form, making use of the animosity caused by economic stagnation post 35ish years of communist rule as well as pre-20th century historical conflict between the different ethnic/religious groups.

Serbia in particular began taking increasingly belligerent positions (politically speaking) towards the other republics driven by Slobodan Milosevic. Serbia was the largest of the ethnic minorities making up the Yugoslav state and Milosevic began using this to justify the formation of a greater Serbia - seeking to assert a nation for all traditionally ethnic Serbian populations.

As this was going on others of the Yugoslav republics also began jockeying for their own sovereignty within their defined ethnic borders. It became apparent to the others that the Serbian wing of the central government would continue to wield the strongest power and to remain within Yugoslavia would be essentially forfeiting some levels of autonomy to the Serbian coalition.

Slovenia was able to make a move and basically secede from Yugoslavia. While this wasn't entirely easy to do the fact that they were outside of the traditional Serbian footprint Milosevic was aspiring to, plus other ties they had to central Europe given their location, allowed them to fairly cleanly break away.

Macedonia eventually also pulled away - similarly they were fairly untangled ethnically and geographically from the other major republics.

Montenegro was majority Serbian by ethnicity so they were largely ok to remain in Milosevic's good graces and become a part of an ethnic-majority ruling block in his new 'Yugoslavia.'

Vojvodina was also more Serbian facing and were folded into the coalition remaining in Yugoslavia. Kosovo on the other hand, being ethnically Albanian, both resisted this set of moves but also found itself a bit on the outside of the Croatian-Bosnian-Serbian conflict being a more distinct ethnic group and located in a more segregated area to the south/east of these other Republics. The scope of this question doesn't need us to go into more detail but suffice it to say the Kosovo wars followed the below wars and marked the true end to the break-up of Yugoslavia.

Now we are down to the 3 problem Republics: Croatia, Bosnia, Serbia. These three states are simultaneously so closely related to each other in common language and history that their people have been intermingling and spreading among the region for centuries. On the other hand, very distinct lines and shared histories created various major antagonizing forces between them (dating back centuries).

Croatians were catholic vs. the Serbs orthodoxy. They were also a bit more culturally and economically west facing, unfortunately including harboring a strong fascist contingent during WWII and committing atrocities towards their none-catholic and non-fascist neighbors during the wars.

The Serbs were orthodox, resisted the fascists (along with a cross ethnic partisan movement) and also felt entitled to areas of shared land due to historical sacrifice they made pushing against Muslim encroachment into the region. They fashioned themselves as protectors of European Christianity who have suffered in their efforts to resist Muslim encroachment.

The Bosnians (citizans of Bosnia the Republic) were seriously just a mix of all of the above - though ethnically speaking the Bosnian-Bosiaks demographic within Bosnia would have been Muslim, converted over the centuries of Muslim expansion and then Ottoman occupation - which again Serbia being on the Eastern border had resisted. But more than just this Muslim population, you had large populations of Serbs and Croats living in Bosnia.

So the whole series of wars came down to Serbia (or serbians in these other republics - often backed by the Yugoslav army which was essentially operating under Serbian/Montenegran control after the other republics declared their independence) trying to sieze regions of the other countries to build their own congrous Serbian state. Croatia also began doing the same in certain border areas - mostly into Bosnia. Bosnia was stuck between these two and just due to its geographic position had a population that was mixed to the point of making it impossible to simply cede territories cleanly based on ethnic or religious grounds. They were literally the melting pot located in the middle of the other 2 Republics.

Instead Serbians and Croatians began 'cleansing' the territories they wanted to incorporate such that a city (for example) that was previously about 33/33/33% in population would become 100% serbian (for example). As mentioned earlier the Yugoslav army at this point was de-facto a Serbian entity and assisted in launching offensives into Bosnia to pursue this cleansing.

This went on for years. And the truly tragic reality is that by the time the final cease-fires occurred and maps could be drawn up the populations themselves had been either murdered or forcefully removed into neater areas to allow for new state boundaries to be established.

TL:DR - The Serbians felt they were entitled to a larger state based upon their large minority population spread into fairly large swatches of Bosnian (and some Croatian) territory. These regions were not 100% Serbian so as the wars got started they began forcefully evicting and in numerous cases, committing mass murder for the sake of creating serbian homogenous areas to annex. There would have been historical animosity by the nationalist serbs towards Muslim Bosnians, though for generations before this time groups in most of these areas (within Bosnia) had been living peacefully amongst each other.

The Croatians also participated, both against the Serbians and against the Bosnians as it suited their own territorial aims.

The Bosnians mounted what minimal defense they were able and had the UN come in as peace-keepers, though the lack of orders to use UN force against the Serbs often led to pretty horrid outcomes under UN supervision.

Edit: Big thanks to u/FeeComprehensive75 & one other (not sure why removed) to provide the final automomous region in the former Yugoslavia (Vojvodina). I have added them to the above comment for completion.