The Corfu Declaration was merely the starting point of the domino effect which would result in the swift creation of a united Slavic state. This Declaration was followed by President Wilson's Fourteen Points on January 8, 1918. President Wilson's speech, points ten and eleven in particular, confirmed Serbia's high international prestige and its good standing among the Allies. Furthermore, Serbia was offered most of Slavonia and almost half of Dalmatia. Croatia and Slovenia on the other hand, found themselves in a no-win situation. They were disliked by the Allies for their alliance with the Central powers and as a result Italy had been promised vast amounts of Croatian and Slovenian land. Therefore, their only alternative was to become strong advocates of the new kingdom. Although they each wanted their own independent nation, at least this union would allow them to salvage some of their land that would otherwise be distributed to the Italians. At the same time, the Croatians and Slovenians could prosper from their alliance with the prestigious and highly regarded Serbs.
Source: NothPark University
http://campus.northpark.edu/history/Web ... ngdom.html
Emphasis is my own.
The creation of the new state in the Balkans was reflected in international peace treaties in which this new state’s name had to appear among the
names of Allied forces - victorious powers of World War I. This place had been intended for Serbia, but by the act of unification Serbia ceased to exist as an international legal entity. Hence, the name of the new state - the State of Serbs, Croats and Slovenes was entered in the peace treaties, and in the Pact of the League of Nations as a founding country.
As I said Croatia had a choice beetween joining Serbia or ceasing to exist, they joined Serbia who accapted them into the kingdom.
Regards,
Serb Tiger