On 1 December 1918, King Alexander formally proclaimed the existence of the Kingdom of Serbs, Croats and Slovenes (Kraljevstvo Slovenaca, Hrvata i Srba - SHS), and a delegation of the National Council of the State of Slovenes, Croats and Serbs signed the act on unification with the Kingdom of Serbia. As soon as it was signed, and for decades to come, this document was interpreted in different ways that led to misunderstanding and fundamental disagreement. While the Serbs considered the new state an extension of Serbia, on the basis of its victory in the just concluded war, the others, primarily (and at that moment almost exclusively) the Croats and the Slovenes, considered it the unification of several nations on an entirely equal foundation and without anyone’s special merit.

The Serbs and the Croats were by far the most numerous nations both in pre-war and post-war Yugoslavia, and lived mixed in many regions. This made it clear that Croat-Serb relations and the Croatian national question would be the most important and difficult challenge of the Yugoslav state and its greatest ethnic problem.

From Belgrade came an ideological picture of the past: foreign powers ostensibly had once in the past dismembered an allegedly unified people and has politically and culturally alienated them, one from the other.  Unity would not only bring liberation to blood brothers from foreign domination; it would also bring about the removal of "deposited" foreign influences and a return to unified national origin.

Some circles in Croatia were thinking the same way; in Croatian society after 1918 there was a powerful conviction that the Yugoslav option, meaning unification with Serbia, was the result of a maturation of cultural and national consciousness at a high level.  "Yugoslavs" were mainly from established urban families. 

Nevertheless, right from the start people had reasons to doubt that the new state would bring a better life. The successive disappointments that many non-Serbs, as well as many Serbs outside of Serbia, experienced before the Second World War brought about such a situation that in April 1941, when the Axis Powers attacked, the country became its easy prey.

In Croatia, for example, for various reasons many Croats greeted the proclamation of the Independent State of Croatia (ISC) on 10 April 1941 with enthusiasm, but disappointment soon set in and sympathy evaporated.

In the following weeks and months the population of Yugoslavia gradually, but increasingly, tilted toward the anti-fascist movement, and even began to actively participate in it, in part because of its attitude on the national question.  After the war, behind the polished exterior of the construction of a new, happy society were many extremely dissatisfied people who had been stripped of their rights and inheritance, or who had lost family members in unprovoked retaliation. All the same, the new government in Yugoslavia was certainly more popular among some people than any other government in Eastern Europe.

There is no doubt that Yugoslavia, as a kingdom and as a socialist state, regardless of what contemporaries may have thought, was a fragile creation.  It was incapable of confronting national aspirations of its peoples in an appropriate manner, so it tried to extinguish them by force, which unavoidably had the contrary effect. 

At the end of the 1980s among the publics of the three strongest republics – Serbia, Croatia and Slovenia – there was a predominant view that all the others in Yugoslavia were developing at the expense of their own republic.  Nevertheless, at the beginning of the 1990s the principle of freedom, not only of individuals but also the free will of a people, decided the fate of the Yugoslav state.