Croatia was the main "aggressor" in the Great Balkan War. Since the CSDS was dominated by Croats and the majority of the army was Croats, they attempted to keep the territory of the CSDS together by force after Bulgaria became the first member state to declare independence.

Bulgaria declared independence in 1988 and immediately aligned itself with SNOR-ist Russia, thus guaranteeing its independence; the Croat-dominated Danubian Peoples' Army (DPA) units stationed there withdrew in a 9-day "war".

In 1989, the son of the Ustasha leader Ante Pavelic, Tomislav, returned from Helvetia to Croatia to re-establish the Ustasha movement which his father led in Croatia during GW2. After GW2 ended and the communists led by Josip Broz won the ensuing civil war and establishing the CSDS, Ante was forced to flee, and he settled in Helvetia, where he died peacefully in 1959. After their return, Tomislav's son Zeljko organised paramilitary units that were quite brutal. Tomislav Pavelic became the leader of the new Ustasha party, and he became close with then-CSDS President Franjo Tudjman.

It was only after Dalmatia declared independence in 1990 that the war started for real. The DPA was sent in by CSDS president Franjo Tudjman, so Dalmatian Territorial Defence units became the Dalmatian Army and fought back. Shortly thereafter Serbia declared independence too, and the Croats were thus fighting on two fronts. The Croats were backed with arms and funds during this time by Germany and Helvetia, and to a lesser extent, Austria. While Croatia was fighting only Dalmatia, a few shipments of arms arrived from SNOR-ist Russia, but after Serbia declared independence these shipments stopped.

In 1995, in the region of Croatia immediately to the east of Osijek and to the north of there was also declared a Serb state called Republika Srpska Slavonija with capital at Vukovar. Subsequently the RSS was annexed to the Serbian Kingdom in 1998 and was retaken by Croatia in 1999 when the Serbs were occupied with Sanjak in the south.

In 1996 Slovenia declared independence, but their state didn't last long, having been quickly crushed by the Croats. Also in 1996 Tudjman died, and Tomislav Pavelic became president and he orchestrated the crushing of Slovenia and the retaking of Serb Slavonia, which had been incorportated into the Kingdom of Serbia in 1998 but had been declared an independent state already in 1995. In 1999 there was a restructuring of the government which resulted in Pavelic becoming a very influential figureheadas regent, and Krunoslav Tomit' was 'elected' president (there were only Ustasha candidates) in 2000.