The basis of Croatia and Serbia forming a union in 1918 is to be found in the complex history of the Yugoslav Committee. The Yugoslav Committee was formed by exiles living outside the Croatian homeland during World War I. The Committee was led by Frano Supilo and Ante Trumbic and included the famous Croatian sculptor Ivan Mestrovic. Each repudiated the Committee within a few years of the founding of Yugoslavia. "Yugoslavs" were Serbian, Croatian and Slovene people who identified themselves with the movement toward a single Yugoslav or South Slavic state. Exiled Yugoslavs living in North America and Britain were the primary supporters of the Yugoslav Committee. Having established offices in London and Paris as early as 1915, the Yugoslav Committee became an active lobby for the cause of a united South Slav state during the First World War.
The concept of a unified South Slavic state had been discussed by Croatian and Slovene intellectuals since the mid-nineteenth century. However, the "Yugoslav Idea" did not mature from the conceptual to practical state of planning. Few of those promoting such an entity had given any serious consideration to what form the new state should take,. Nevertheless, the Yugoslav Committee issued a manifesto calling for the formation of such a South Slavic state on May 12, 1915. The document, like the rhetoric of those who produced it, was vague concerning the form and system of government. It received little official recognition.
At the same time Serbia, led by Nikola Pašić's People's Radical Party, saw the "Yugoslav" concept as a useful tool in the long sought development of a "Greater Serbia. As the War dragged on, the Allies began to think of the concept of Yugoslavia as a blocking force in the Balkans to counter future German expansionism. Although no formal agreement was announced until July 1917, the Yugoslav Committee and the Serbian Government-in-Exile worked hand-in-hand from November 1916 onward. On July 20, 1917 the Serbian government and the Yugoslav Committee issued the text of an agreement known as the Corfu Declaration which called for the formation of a multi-national state. The document was deliberately mute as to whether the government would take the form of Western-oriented Croatia or of the Eastern-oriented Serbia. The vast majority of the Serbian, Croatian and Slovene people had no knowledge of the declaration made by a small group of exiled intellectuals and the Serbian Government-in-Exile. Nonetheless, the signers claimed to speak for all South Slavic peoples and the Corfu Declaration became the justification claimed by Serbia for the forced unification of Croatians and Slovenes under the Serbian crown.
Overlooked was Congress held just blocks away on the very next day. This was the Congress of Stjepan Radic's Croatian Peasant Party attended by almost three thousand elected delegates from every part of Croatia . The Peasant Party was the largest and most popular party in Croatia at that time and would remain so during the period between the Wars. It won absolute majorities in every subsequent election . This Congress assailed the National Council as arbitrary and unconstitutional and unanimously adopted a resolution calling for a "Neutral and Peasant Republic of Croatia." Following this Congress, there were huge demonstrations in the streets of Zagreb supporting independence .
Zagreb's brief jubilation quickly changed to the sober realization that Croatia would again be ruled from a foreign capital as Italian, French and French African forces invaded from the west and Serbian troops invaded from the east.
On December 1, 1918, Serbian Prince Alexander announced the formation of the Kingdom of Serbs, Croats and Slovenes, with a Serbian King ruling from the Serbian capital of Belgrade. Despite the neutral sounding name, the country was called Yugoslavia by the diplomatic community almost from the beginning. Ironically, at the Paris Peace Conference the Yugoslav delegation openly insisted that it be known as the "Serbian Delegation."
It is interesting that the greatest promoters of creating a state of the Southern Slavs, i.e. the idea of Yugoslavia, were the Croats (Josip Juraj Strossmayer on the first place), but they did not conceive of it as the centralized, Serb-dominated state. Their aim was to preserve the Croatian national identity and the sovereignty of Croatia and to organize the new state of South Slavs on a confederative basis.
That is why the Kingdom of the Serbs, Croats and Slovenes, established in 1918, did not obtain the confirmation and permission of the Croatian Parliament. This state, created in 1918 from the Austro-Hungarian part, (Slovenia, Croatia, Vojvodina, Bosnia and Herzegovina) and Serbia and Montenegro, which were opposing sides during the First World War (1914-1918), contained a germ of numerous future conflicts. It was composed of different traditions, religions, nations, languages and scripts.
At that time the region of Vojvodina did not include Srijem (the territory between rivers Sava and Danube), that before 1918 belonged to Croatia. Vojvodina belonged to Hungary before 1918.
The idea of Yugoslavia was in fact the best opportunity for Serbian nationalists to create the Greater Serbia, which was completed in 1918 according to the 1844 secret programme. Montenegro joined Serbia in 1918. The independence of Montenegro was regained in 1945 within the Tito's Yugoslavia.
The whole property of the Austro-Hungarian state and booty was confiscated by the Serbian authorities. Immediately after 1918 all the leading positions in the army were seized by Serbian officers, who treated Croatia as a hostile territory in the common state (it was publicly declared in 1919). On the other hand, it was presented to Europe as if the Croats had entered willingly the union with Serbia.
The Serbian legislature, juridical and military 19th century law was simply implemented into the new state without changes and without consultations with the Croats. It resulted in unbearable terror and persecutions of Croatian peasants and intellectuals. Croatian teachers were retired and persecuted.
Equally difficult was the economic terror of the Belgrade government. The Croats were not proportionally represented in the government and diplomatic corps. The old currencies - Serbian dinars and Croatian (Austrian) crowns, which in 1918 had the same value, were in 1919 changed for the new dinar in the following ratio: 1 dinar = 4 crowns
On the other hand,
Persecutions of the Muslims by the Serbs resulted in their massive emigration to Turkey soon after the foundation of Kingdom of Serbs, Croats and Slovenes in 1918, where Serbia was the leading and privileged nation. The same happened to several hundred thousand Muslims soon after the Second World War.
The concept of “Greater Serbia” in Yugoslavia was put in practice during the early 1920s, under the Yugoslav premiership of Nikola Pasic. Using tatics of police intimidation and vote rigging , he diminished the role of the oppositions (mainly those loyal to his Croatian rival, Stefan Raditch ) to his government in parliament, creating an environment to centralization of power in the hands of the Serbs in general and Serbian politicians in particular.
One of the most outstanding and most popular personalities in the Croatian political history was Stjepan Radic (1871-1928), the leader of the Croatian Peasant Party, assassinated in the Yugoslav parliament in Belgrade (capital of present Serbia) in 1928 together with his colleagues. The assassination was organized at the Royal court in Belgrade. Radic strived to renew the Croatian sovereignty and the economic and cultural emancipation of Croatia. He wanted the state of the Southern Slavs to be reorganized on confederative basis, without Serbian hegemony.
The culmination of the Serbian police terror took place during the personal dictatorship of king Aleksandar Karađorđević since 1929. One of the historical documents from that period, showing "methods" of the Serbian police and administration, is a bill on 13 dinars and 15 paras charged to a Croatian family in 1934 for five bullets fired at the father, who was sentenced to death. The families were persuaded even to pay the "expenses" of the execution within eight days, under the threat of confiscation of their property. Croatian archbishop Alojzije Stepinac reported about this event to the French diplomat Ernest Pezet in 1935. Belgrade also made use of the world economic crises in 1929 to destroy the Croatian banking system, which had been the strongest in Yugoslavia.
Croatian scientists were also victims of the Serbian terror. Milan Šufflay, historian of international reputation known by his numerous scientific contributions, especially in the field of albanology, was assassinated by a steel rod on a street in the center of Zagreb in 1931. After the dramatic events that followed, Albert Einstein and Heinrich Mann sent an appeal to the International League of Human Rights in Paris to protect Croats from the terror and persecutions of the Serbian police. It was also published in the New York Times (6th May 1931). As we learn from this letter, the newspapers in Zagreb were not allowed to report about Sufflay's activity; it was not allowed to attach a half-mast flag on the main building of the University of Zagreb in his honour; the time of the funeral could not be announced publicly, and even condolence messages were not allowed to be telegraphed. In their letter Einstein and Mann hold the Yugoslav king Aleksandar explicitly responsible for the state terror over the Croats. The letter concludes that it should not be tolerated that killings be allowed as a means to achieve political goals. "We should not allow killers to be promoted as national heroes." The king himself was assassinated by a Macedonian patriot in Marseille in 1934 (there are indications that there was a collaboration of the Macedonian Revolutionary Organization with the Ustasha organization).
An extremely valuable account on the terrorist methods of the Pan-Serbs in Yugoslavia between the two World Wars has been written by Henri Pozzi, a brave French diplomat (his mother was English) and a close witness, in his book Black Hand over Europe, London, 1935. "The Black Hand" is the name of the Pan-Serbian secret terrorist organization, very close to the Royal court in Belgrade. It was the "Black hand" that organized the assassination of the Austrian archduke Ferdinand Habsburg in Sarajevo in 1914, which meant the beginning of the First World War
All the best posts in Croatia were occupied by the Serbs. Around 1930 the situation in Croatia was to following:
The tendency of administrative parcelization of Croatia that started in 1922 was revised by the establishment of the autonomous Croatia - Banovina of Croatia - in 1939. It also included parts of Bosnia and Herzegovina.