[1] The basis of State of Slovenes, Croats and Serbs and Kingdom of Serbia forming a union in 1918 is to be found in the complex history 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 World War I.
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.
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 Croatian Parliament (Sabor) met in Zagreb on October 29, 1918 to declare "the Kingdom of Croatia, Slavonia and Dalmatia" to be a free and independent state.
All major parties from the Croatian Parliament had named representatives into the new National Council of Slovenes, Croats and Serbs that had been formed in early October 1918, and which in turn took control over most of the Austro-Hungarian possessions inhabited by South Slavs.
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.
This state, created in 1918 from Austro-Hungarian part, (Koruška, Štajerska, Kranjska, Istra, Dalmatia, Croatia - Slavonia, 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.
Using tactics of police intimidation and vote rigging,[7] he diminished the role of the oppositions (mainly those loyal to his Croatian rival, Stjepan Radić) 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.
The assassination of Stjepan Radić caused a major political crisis and the latter half of 1928 was marked with demonstrations and recriminations, with the Croats and prečani Serbs united in demands for federalization.
One of the historical documents from that period, showing "methods" of the Serbian police and administration, is a bill on 13 dinars and 15 paras charged to a Croatian family in 1934 for five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[citation needed].
Belgrade also made use of the world economic crises in 1929 to destroy the Croatian banking system, which had been the strongest in Yugoslavia[citation needed].
Because of this, 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"[citation needed].
In their letter Einstein and Mann held the Yugoslav king Aleksandar explicitly responsible for the state terror over the Croats[citation needed].
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 French diplomat and a close witness, in his book Black Hand over Europe, London, 1935, referring in the title to the "Black Hand", the Pan-Serbian secret terrorist organization, very close to the Royal court in Belgrade.