Croatia and Serbia maintain diplomatic relations established between Croatia and the Federal Republic of Yugoslavia (of which Serbia is considered sole legal successor) in 1996.

Croatia and Serbia have a complicated relationship marked by a variety of bilateral issues. The relations, established following the dissolution of Yugoslavia and the Croatian War of Independence, are functional but cool, stemming from historic conflicts and divergent political ideologies. Croatian and Serbian, official languages in Croatia and Serbia respectively, are mutually intelligible standard varieties of the Serbo-Croatian language.

History 19th century

With the nation-building process in the mid-19th century, the first Croatian–Serbian tensions appeared. Serbia's Prime Minister Ilija Garašanin's Načertanije (1844) claimed some lands that were inhabited by Bulgarians, Macedonians, Albanians, Montenegrins, Bosniaks, Hungarians, Croats and Slovenes as part of Greater Serbia. Garašanin's plan also envisioned methods of spreading Serbian influence in the claimed lands. He proposed ways to influence Croats, who Garašanin regarded as "Serbs of Catholic faith". Serbian linguist Vuk Karadžić considered Croatians, who spoke Shtokavian dialect, "Catholic Serbs". Croatia was at the time a kingdom in the Habsburg monarchy, with Dalmatia and Istria being separate Habsburg Crown lands. Croatian thinker and politician Ante Starčević, an advocate of Croatian unity and independence, who was both anti-Habsburg and anti-Serbian in outlook, envisioned the creation of Greater Croatia that would include territories inhabited by Bosniaks, Serbs, and Slovenes, considering Bosniaks and Serbs to be Croats who had been converted to Islam and Orthodox Christianity, while considering the Slovenes to be "mountain Croats". Starčević, who in 1861 co-founded a nationalist and irredentist Party of Rights, argued that the significant Serb presence in territories claimed by Greater Croatia was the result of recent settlement, encouraged by Habsburg rulers, and the influx of groups like Vlachs who converted to Orthodox Christianity and came to identify themselves as Serbs. Starčević admired Bosniaks because in his view they were Croats who had adopted Islam in order to preserve the economic and political autonomy of Bosnia and Ottoman Croatia. After Austria-Hungary occupied Bosnia and Herzegovina in 1878 and Serbia gained its independence from Ottoman Empire, Croatian and Serbian relations deteriorated as both sides had pretensions on Bosnia and Herzegovina.

Early 20th century and the World War I

In 1902, anti-Serb riots in the Kingdom of Croatia-Slavonia were incited by a re-publication by the Zagreb-based Serb Independent Party of an article authored by a Serb Nikola Stojanović that was titled Srbi i Hrvati ("Serbs and Croats"), also known as Do istrage vaše ili naše ("Till the Annihilation, Yours or Ours"). Stojanović denied the existence of the Croatian nation and forecast the result of the "inevitable" Serbian–Croatian conflict: That combat has to be led till the destruction, either ours or yours. One side must succumb. That side will be Croatians, due to their minority, geographical position, mingling with Serbs and because the process of evolution means Serbhood is equal to progress.

In 1905, the Croat-Serb Coalition was formed, a political alliance between Serbs and Croats in Austria-Hungary which advocated for South Slavic unification and viewed German expansion as the biggest threat to it. In the 1906 Croatian parliamentary election they won a majority of seats in the Parliament of the Kingdom of Croatia-Slavonia and dominated Croatian politics until the end of the World War I.

In World War I, ethnic Croats fought in the Austro-Hungarian Army against the Kingdom of Serbia, while Croatian general Ivan Salis-Seewis was a military governor of occupied Serbia. Croatian troops in the Austro-Hungarian Army committed a number of war crimes against the Serbs, especially in Mačva region, where according the civilian population was subjected to a wave of atrocities. Between 3,500 and 4,000 Serb civilians were killed in executions and acts of random violence by marauding troops. On the other hand, there were examples of Croat POWs volunteered to fight on the Macedonian front with the Serbian Army.

Kingdom of Yugoslavia

On 29 October 1918, the Croatian Parliament declared independence from Austria-Hungary and decided to join the newly formed State of Slovenes, Croats and Serbs which on 1 December 1918 entered a union with the Kingdom of Serbia and formed the Kingdom of Serbs, Croats, and Slovenes. Initial Croatian zeal for the new state faded away as the republican view of a new state was ignored, especially since the concept of Greater Serbia was put in practice during the early 1920s, under the Yugoslav premiership of Nikola Pašić. Using tactics of police intimidation and vote rigging,Balkan Politics, Time magazine, 31 March 1923 he diminished the role of the oppositions (mainly those loyal to his Croatian rival, Stjepan Radić) to his government in parliament,Elections, Time magazine, 23 February 1925 creating an environment for centralization of power in the hands of the Serbs in general and Serbian politicians in particular.The Opposition, Time magazine, 6 April 1925 Police violence further alienated Croats, who began to ask for their own state. In 1928, Stjepan Radić and five other Croat politicians were shot in the National Assembly by a Serb deputy Puniša Račić, enraged by continuous Croatian claims that they were "exploited by Serbs and that Serbs are treating them like a colony." This led to the royal dictatorship of King Alexander I in 1929. The dictatorship formally ended in 1931 when the king imposed a more unitarian constitution and changed the name of the country to Yugoslavia. In 1934, King Alexander I was assassinated in Marseille by a member of the pro-Bulgarian Internal Macedonian Revolutionary Organization, which received assistance from the Croat Ustaše led by Ante Pavelić. The Croatian Peasant Party, led by Vladko Maček (who succeeded Radić), continued to advocate federalization of Yugoslavia, resulting in the Cvetković–Maček Agreement of 1939 and the autonomous Banovina of Croatia.

World War II

In April 1941, Yugoslavia was occupied by Germany and Italy who created a puppet-state called the Independent State of Croatia which was governed by the pro-Axis Ustaša organization. The Ustašas sought to create ethnically pure Greater Croatia by cleansing Serbs as well as Jews and Roma from its territory, under its policy where the one-third of the Serb population faced a choice of extermination, expulsion or forced conversion to Roman Catholicism. The Ustaša regime systematically murdered around 300,000–350,000 Serbs, as a part of a genocide campaign. Approximately 100,000 people, primarily Serbs, Roma and Jews and political dissidents were murdered in Jasenovac concentration camp alone.

The predominantly Serb Chetniks, a Yugoslav royalist and Serbian nationalist movement and guerrilla force, engaged in war crimes and ethnic cleansing of Muslims and Croats in order to establish a Greater Serbia, while also supporting the reinstatement of a Serbian monarchy. Some historians view these crimes as constituting genocide. Estimates on the number of Muslims and Croats deaths caused by the Chetniks in Croatia and Bosnia and Herzegovina range from 47,000 to 65,000. Following the victory of Yugoslav Partisans, who were led by Croatian communist Josip Broz Tito, the Ustašas and the Chetniks were defeated. Yugoslav communists abolished the monarchy and established one-party socialist republic and a federation governed by the League of Communists of Yugoslavia.

Socialist Yugoslavia

During the period of socialist Yugoslavia relations between two largest constituent republics, SR Serbia and SR Croatia, were shaped by the complex dynamics of a multi-ethnic federation under the unifying ideology of "Brotherhood and Unity", promoted by Josip Broz Tito and the League of Communists of Yugoslavia. During this period, their relationship was characterized by cooperation within the federal framework but it was also strained due to historical grievances, political and economic interests, as well as linguistic issue.

The memory of World War II, particularly the atrocities committed by the Ustaša regime against Serbs lingered in Serbian collective memory. Conversely, Croats harbored resentment over Serbian dominance in the pre-war Kingdom of Yugoslavia where Serbs held disproportionate influence in government and military.

A major source of tension was the debate over the structure of the Yugoslav federation. Serbia often favored a more centralized Yugoslavia, which aligned with perceptions of Serbian dominance. Croatia, on the other hand, pushed for greater autonomy for the republics, reflecting Croatian desires for cultural and political self-determination. Croatia, with its developed industrial base and Adriatic coastline, was a key economic driver, contributing significantly to Yugoslavia’s economy. Serbia, while significant in industry and agriculture, held political and administrative influence as the largest republic and home to the federal capital, Belgrade.

The status of the Serbo-Croatian language was a point of contention. While officially treated as a single language with two variants (Serbian using Cyrillic, Croatian using Latin script), Croats increasingly emphasized linguistic distinctiveness, reflecting broader cultural efforts to assert Croatian identity within the federation. In 1967, Croatian authors and linguists published a Declaration on the Status and Name of the Croatian Standard Language demanding equal treatment for their language.

The most significant strain in Serbia-Croatia relations during socialist Yugoslavia was the Croatian Spring, a political and cultural movement in Croatia from 1967 to 1971. Croatian intellectuals and politicians, including members of the League of Communists of Croatia, demanded greater autonomy, economic control, and recognition of Croatian language and identity. They criticized perceived Serbian hegemony in federal institutions and the redistribution of Croatia’s economic resources to poorer regions. Serbia viewed the movement as a revival of Croatian nationalism and a threat to Yugoslav unity. Tito suppressed the movement in 1971, purging its leaders, Savka Dabčević-Kučar and Miko Tripalo, and imprisoning many activists, which deepened resentment in Croatia.

These tensions were managed under Tito’s strong and unifying leadership but resurfaced in the 1980s as economic crises and nationalist movements eroded the Yugoslav project. In Serbia, the rise of Slobodan Milošević in the late 1980s, with his nationalist rhetoric and focus on protecting Serbs across Yugoslavia, alarmed Croatia. Milošević’s moves to centralize power were seen in Croatia as a return to Serbian dominance. In Croatia, nationalist sentiments re-emerged, fueled by economic frustrations and fears of Serbian centralization. Figures like Franjo Tuđman began advocating for Croatian sovereignty, contributing to the emergence of conflict as Yugoslavia unraveled.

Breakup of Yugoslavia and the Croatian War of Independence