(Given 12% of the population of Croatia, 600,000 is more Serbs than were living there to start with, methinks).
Croatia has census information on the web, but it doesn't have an ethnic breakdown. I'll wait for that to come out, rather than relying on 'supposedly', no offense.
The Muslims also helped the Croats ethnically cleanse the Serbs. They admit it:
Six years ago, at the height of the conflict in Bosnia, Ms. Decter asked me to translate testimonies given by a group of Bosnian Muslim prisoners that were brought to Boston (Allston) the previous day. They gave a harrowing account about being kept for months in a dark underground military tunnel in Hercegovina. The detainees described the torture and killings they witnessed, and talked about their transfer to the island of Badia and their consequent release to the USA. When they asked me to translate, the American Jewish Congress was not aware that the prisoners were not held in Serbian but in a Croatian detention camp. When did the Serbs put them in prison? the AJC representative repeatedly asked me. They were in a Croatian camp, I kept repeating. They were tortured by the Croats not Serbs. This came to them as a complete surprise.
To make the situation more ironic, the former Muslim prisoners described in detail how at the beginning of the conflict, they, together with the Croatians from the area, ethnically cleansed and killed all the Serbs from the town of Chaplina and the surrounding villages. They showed no remorse for their actions, but told me the story embittered by the betrayal of their former Croatian allies. The American Jewish Congress office in Boston never released the tapes, that are probably still in their possession. Since the tapes did not show the Serbs as being solely responsible for the civil war in Bosnia, Sheila Decter must have decided that it is better not to ruin her imaginary account of the conflict in Bosnia.