The Croat-Serb Coalition (Serbo-Croatian: Hrvatsko-srpska koalicija/Хрватско-српска коалиција) was a major political alliance in Austria-Hungary during the beginning of the 20th century that governed the Croatian lands (crownlands of Croatia-Slavonia and Dalmatia). It represented the political idea of a cooperation of Croats and Serbs in Austria-Hungary for mutual benefit. Its main leaders were, at first Frano Supilo and then Svetozar Pribićević alone.
The Coalition governed the Croatian lands from 1903 until the dissolution of the Dual Monarchy in 1918 and the Yugoslav unification, when it was by large integrated into the Yugoslav Democratic Party.
The previous incarnation of Croat-Serb cooperation in the historical Croatian lands under Austro-Hungarian rule had happened sixty years earlier in the Illyrian movement, but that idea came to an abrupt end with the revolution of 1848.
The underlying reason for the formation of the Coalition in the early 1900s was the mass realization that the Hungarian and Austrian governments as well as the Italian irredentists all profit from the divisions between the Croats and the Serbs. This became particularly apparent following the popular demonstrations against the Croatian ban Khuen Hedervary in 1903, where the masses of Croat peasants were joined by Serb peasants, and achieved a greater effect.
The Coalition itself originated in the Resolutions of Rijeka and Zadar of October 1905, wherein the groups of individual Croat and Serb parliamentary representatives formulated requests for the improvement of Croat and Serb national interests, respectively, focused on the integration of Dalmatia with Croatia-Slavonia and the elevation of the country's position within the monarchy.
The parties which joined the Coalition initially included:Croatian Party of Rights
Croatian Progressive Party
Serb People's Independent Party
Serb People's Radical Party
By this time, the Party of Rights had also included members of the Croatian People's Party, who had previously split from the Magyarized mainstream faction. The Social-Democrats and Radicals would later break away from the Coalition.
On December 11, 1905, the Coalition representatives published their political programme. In the Croatian parliamentary election, 1906 they won a majority of seats in the Croatian Parliament.
In 1908, the Coalition won the election again, but it also came under attack from the Vienna Imperial Court, which accused its leadership of grand treason. In 1909, 53 members of the Serb Independent Party were actually put on trial for collaboration with Serbia in a conspiracy to unite all South Slavs into a single state. In this politically motivated trial, known as the Agram Trial, with the main witness a police agent provocateur, the defendants were found guilty with flimsy evidence and given extended prison sentences. However, after the Coalition decided to form a political alliance with the Austro-Hungarian authorities, their members were all pardoned. This came at a cost of having to marginalize their leader Frano Supilo and having to temper their criticism of the government in the Kingdom of Hungary. Svetozar Pribićević became the new leader and closed a formal agreement with the government in 1913.
The Coalition continued to win elections in 1910 and 1913. It dominated Croatian politics throughout World War I while continuing to support the state of Austria-Hungary. Nevertheless, the leaders of the Coalition participated in the Yugoslav Committee during World War I.
When the war ended and the State of Slovenes, Croats and Serbs was formed, the Coalition fielded 12 representatives in the National Council of the State.
Later the Kingdom of Serbs, Croats and Slovenes was formed, and the party dissolved and its former members mostly became advocates of the new government in Belgrade.
The Peasant-Democratic Coalition led by Stjepan Radić and Svetozar Pribićević (later Vladko Maček alone) during the Kingdom of Yugoslavia is generally seen as a recreation of the idea, from 1927 until the country's destruction in World War II in 1941.