Ideologically, Supilo became the proponent of a
Yugoslav unitarism. He held that
Croats and
Serbs (and later
Slovenes) were a single people with three "tribes". This idea was shared by many of his contemporaries, based on the late national awakening of Croats, Serbs and Slovenes (not to mention
Macedonians,
Bosniaks and
Montenegrins), the territorial closeness of Serbs and Croats, and the facts that the two peoples have very similar official languages. In practice, it meant the creation of a coalition that virtually recognized Serbs as a political entity in Croatia for the first time in its history. An even stronger turn was the negotiation of the coalition and Supilo with the Hungarian and Italian politicians from
Austria-Hungary. It was a brave and innovative move, but proved unsuccessful. Supilo played on the temporary conflict between the Vienna court and the Italian
irredentists and Hungarian imperialists, the traditional Croatian opponents who claimed their rights to some Croatian lands. He wanted to turn them into Croatian allies in the fight for the general democratization of the monarchy, which he believed would profit all peoples. But he estimated wrongly: the Italian and Hungarian imperialism was so deeply entrenched that it fell only after the world wars. As for Serbian nationalism, Supilo did not realize how stubborn were Serbian territorial claims over large parts of
Bosnia and Herzegovina and
Croatia. When the
Croat-Serb Coalition won the elections of 1906, Supilo became a representative in the
Croatian parliament and the leading figure of the Coalition. When the Hungarian parliament in spring 1907 decided that Hungarian would be the official language of the Croatian railroads, Supilo led the Croatian opposition in obstructing actions in the Hungarian parliament. He waged a strong campaign against the
ban Levin Rauch, who supported the Hungarians. Supilo also initiated the fundamental constitutional issues on the status of Croatia. His radical attitude brought him in conflict with the Coalition leadership, which promoted a more careful policy with a view of another mandate. Supilo tried to bring Croatian policy to the forefront of the South Slavs of Austria-Hungary in their fight for unification, while the Serbian part of the Coalition and some of its Croatian members wanted to harmonize Croatian policy with the Kingdom of
Serbia, which would bring Croatia to a politically inferior position in relation to Serbia. At the time of the "High Treason Trial" (1909), the politically motivated trial against the Serb public officials in the monarchy, initiated by the Viennese court because of the crisis around the annexation of Bosnia and Herzegovina in 1908, Supilo advocated a strong support of Croats to the endangered Serbs to preserve the harmony of Croats and Serbs based on the principles from the Rijeka Resolution and the
Zadar Resolution.
Heinrich Friedjung tried to discredit Supilo by forging documents that said Supilo was working on behalf of Serbia.
Thomas Masaryk proved they were forgeries at a trial in 1909. Supilo left the
Croat-Serb Coalition in 1910. ==World War I==