The Yugoslav army invaded and annexed one third of Croatia in 1991. Infamously they destroyed Vukovar and bombarded Dubrovnik.
With the Yugoslav army, the "Krajina" Serbs enthusiastically ethnically cleansed hundreds of thousands of Croats, killing up to 20,000 of them.
They shelled Croatian cities from the occupied areas, ethnically cleansed the remaining Croats and even invaded Bosnia to attack the UN safe haven of Bihac.
The Croatian offensives liberated Croatian territory and saved Bihac from a fate similar to that of Srebrenica.
The "Krajina" Serb leadership ordered their population to leave. Considering their horrific crimes it was hardly surprising they complied. Nor did they wish to be around when many of their victims were dug up, over 3,000 to date.