Detaching from Austria-Hungary, Croatia became part of the formation of the Kingdom of Slovenes, Croats and Serbs, which in 1929 became known as Yugoslavia.
The key to understanding this problem is that the radical political stream was not in favor of the unification with Serbs and Slovenes, but Croatia did unify voluntarily under a Serb king. A radical Croat leader Stjepan Radic was assassinated by Serbs, because of the constant bashing of the Serb role in WW1 (50% of the Serb male population, or 26% of the total, was killed in WW1). This was followed by the assassination of the Serb king by Croats.
Nationalism exploded, Croatia's goal was independence through Yugoslavia and not remain in it, so in 1941 it proclaimed itself independent under the protectorate of Nazi Germany. Besides independence, Croatia was cleansing itself from Jews, Gypsies and Serbs.
http://www.youtube.com/watch?v=pWGyTPu6UDE
In 1945 the war ended and Croatia, becoming again part of Yugoslavia, became a socialist republic, but in 1991, after firstly seeking confederate status, it grabbed its chance with the fall of the Soviet Union and unification of its mother Germany, declaring unilateral independence and degrading Croat Serbs from legal constitutive into legal minority. This was followed with a war against Croat Serbs and Yugoslav military forces, which was supported by Germany and USA in their destruction of Yugoslavia.
http://www.youtube.com/watch?v=nPGlBZ69VlA
The core relationship of Croatia with Yugoslavia can be described with one word: "Ustasha"... Basically, it is all about independence and extermination of Serbs in Croatia. Serbs were once 22% and today are 4% in Croatia.
http://www.youtube.com/watch?v=Prel1TPbo1I&feature=watch_response_rev
It is important to have in mind that Croatia is the boundary of the Vatican...
This pretty much sums it up:
http://www.youtube.com/user/mstreamm?feature=mhum#p/f/9/8C2_KHeaYgE
Edit:
This is the answer "offensiv" gave to a question about Serbs being in Croatia. As we can, see "offensiv" is full of blind hatred:
"Flippy is correct but I just wanted to add that Operation Storm did not expel the Serbs. They were given orders sent from Belgrade to evacuate as they had no desire to live in a free and independent Croatia,"
offensiv is either a teenager or a teenager in a body of an 38 year old idiot. I'll post this answer of his to my facebook.