As the Austro-Hungarian Empire collapsed, the territory of Syrmia united with Serbia on 24 November 1918. Just a day later on 25 November 1918 Grand National Assembly of Serbs, Bunjevci and other Slavs in Banat, Backa and Baranja declared the unification of Banat, Backa and Baranja to the Kingdom of Serbia. On 26 November 1918, the Podgorica Assembly deposed the House of Petrovic-Njegos and united Montenegro with Serbia.

On 1 December 1918, in Belgrade, Serbian Prince Regent Alexander Karadjordjevic proclaimed the Kingdom of the Serbs, Croats, and Slovenes, under King Peter I of Serbia.

The new kingdom was made up of the formerly independent kingdoms of Serbia and Montenegro (Montenegro having been absorbed into Serbia the previous month), and of a substantial amount of territory that was formerly part of Austria–Hungary, the State of Slovenes, Croats and Serbs.

The Kingdom of the Serbs, Croats, and Slovenes bordered Italy and Austria to the northwest, Hungary and Romania to the north, Bulgaria to the east, Greece and Albania to the south, and the Adriatic Sea to the west. Almost immediately, it ran into disputes with most of its neighbours. Slovenia was difficult to determine, since it had been an integral part of Austria for 400 years. The Vojvodina region was disputed with Hungary, Macedonia with Bulgaria.

The Dalmatian port city of Zadar and a few of the Dalmatian islands were given to Italy. The city of Rijeka was declared to be the Free State of Fiume, but it was soon occupied, and in 1924 annexed, by Italy, which had also been promised the Dalmatian coast during World War I, and Yugoslavia claiming Istria, a part of the former Austrian Littoral which had been annexed to Italy, but which contained a considerable population of Croats and Slovenes.

The formation of the constitution of 1921 sparked tensions between the different Yugoslav nationalities. Some Croatian politicians opposed the 1921 constitution and over time grew increasingly hostile towards the Yugoslav government that they saw as being centralized in the favour of Serb hegemony over Yugoslavia.

On 28 June 1921, the Vidovdan (St Vitus’s Day) Constitution was passed, establishing a unitary monarchy. The pre–World War I traditional regions were abolished and 33 new administrative oblasts (provinces) ruled from the center were instituted. During this time, King Peter I died (16 August 1921), and the Prince-Regent succeeded to the throne as King Alexander I.

In the early 1920s, the Yugoslav government of prime minister Nikola Pasic used police pressure over voters and ethnic minorities, confiscation of opposition pamphlets and other measure to rig elections. This was ineffective against the Croatian Peasant Party, whose members continued to win election to the Yugoslav parliament in large numbers, but did harm the Radicals’ main Serbian rivals, the Democrats.

Stjepan Radic, the head of the Croatian Peasant Party, was imprisoned many times for political reasons. He was released in 1925 and returned to parliament.

In the spring of 1928, Radic and Svetozar Pribicevic waged a bitter parliamentary battle against the ratification of the Nettuno Convention with Italy. In this they mobilised nationalist opposition in Serbia but provoked a violent reaction from the governing majority including death threats. On 20 June 1928, a member of the government majority, the Serb deputy Punisa Racic, shot five members of the Croatian Peasant Party, including their leader Stjepan Radic, after Radic refused to apologize for earlier offense in which he accused Racic of stealing from civilian population. Two died on the floor of the Parliament while the life of Radic hung in the balance.

The opposition now completely withdrew from parliament, declaring that they would not return to a parliament in which several of their representatives had been killed, and insisting on new elections. On 1 August, at a meeting in Zagreb, they renounced 1 December Declaration of 1918. They demanded that the negotiations for unification should begin from scratch. On 8 August Stjepan Radic died.

Kingdom of Yugoslavia

On 6 January 1929, using as a pretext the political crisis triggered by the shooting, King Alexander abolished the Constitution, prorogued the Parliament and introduced a personal dictatorship. He changed the name of the country to “Kingdom of Yugoslavia”, and changed the internal divisions from the 33 oblasts to nine new banovinas on 3 October. A Court for the Protection of the State was soon established to act as the new regime’s tool for putting down any dissent. Opposition politicians Vladko Macek and Svetozar Pribicevic were arrested under charges by the court. Pribicevic later went into exile, whereas over the course of the 1930s Macek would become the leader of the entire opposition bloc.

Immediately after the dictatorship was proclaimed, Croatian deputy Ante Pavelic left for exile from the country. The following years Pavelic worked to establish a separatist organization, the Ustase, allied with the Internal Macedonian Revolutionary Organization (IMRO) against the state.

In 1931, Alexander decreed a new Constitution which made executive power the gift of the King. Elections were to be by universal male suffrage. The provision for a secret ballot was dropped, and pressure on public employees to vote for the governing party was to be a feature of all elections held under Alexander’s constitution. Further, half the upper house was directly appointed by the King, and legislation could become law with the approval of one of the houses alone if also approved by the King.

Croat opposition to the new régime was strong and, in late 1932, the Croatian Peasant Party issued the Zagreb Manifesto which sought an end to Serb hegemony and dictatorship. The government reacted by imprisoning many political opponents including the new Croatian Peasant Party leader Vladko Macek. Despite these measures, the Croats continued calling for a solution to what was called the “Croatian question”. In late 1934, the King planned to release Macek from prison, introduce democratic reforms, and attempt find common ground between Serbs and Croats.

However, on 9 October 1934, the King was assassinated in Marseille, France by a Bulgarian activist of IMRO, in a conspiracy with the Croatian extreme nationalist Ustase organisation.

In 1934 the French Foreign Minister Louis Barthou had attempted to build an alliance meant to contain Germany, consisting of France’s allies in Eastern Europe like Yugoslavia together with Italy and the Soviet Union. The long-standing rivalry between Benito Mussolini and King Alexander had complicated Barthou’s work as Alexander complained about Italian claims against his country together with support for Hungarian revisionism and the Croat Ustase terrorist group. As long as France’s ally Yugoslavia continued to have disputes with Italy, Barthou’s plans for an Italo-French rapprochement would be stillborn. During a visit to Belgrade in June 1934, Barthou promised the King that France would pressure Mussolini into signing a treaty under which he would renounce his claims against Yugoslavia. Alexander was skeptical of Barthou’s plan, noting that there were hundreds of Ustase being sheltered in Italy and it was rumoured that Mussolini had financed an unsuccessful attempt by the Ustase to assassinate him in December 1933. Mussolini had come to believe that it was only the personality of Alexander that was holding Yugoslavia together and if the King were assassinated, then Yugoslavia would descend into civil war, thus allowing Italy to annex certain regions of Yugoslavia without the fear of France. However, France was Yugoslavia’s closest ally and Barthou invited Alexander for a visit to France to sign a Franco-Yugoslav agreement that would allow Barthou to, in his words, “go to Rome with the certainty of success”.

Because Alexander’s eldest son, Peter II, was a minor, a regency council of three, specified in Alexander’s will, took over the new King’s royal powers and duties. The council was dominated by the 11-year-old King’s first cousin once removed Prince Paul.

In the late 1930s, internal tensions continued to increase with Serbs and Croats seeking to establish ethnic federal subdivisions. Serbs wanted Vardar Banovina (later known within Yugoslavia as
Vardar Macedonia), Vojvodina, Montenegro united with the Serb lands, and Croatia wanted Dalmatia and some of Vojvodina. Both sides claimed territory in present-day Bosnia and Herzegovina populated by Bosniak Muslims. The expansion of Nazi Germany in 1938 gave new momentum to efforts to solve these problems and, in 1939, Prince Paul appointed Dragisa Cvetkovic as Prime Minister, with the goal of reaching an agreement with the Croatian opposition. Accordingly, on 26 August 1939, Vladko Macek became Deputy Prime Minister of Yugoslavia and an autonomous Banovina of Croatia was established with its own parliament.

These changes satisfied neither Serbs who were concerned with the status of the Serb minority in the new Banovina of Croatia and who wanted more of Bosnia and Herzegovina as Serbian territory. The Croatian nationalist Ustase were also angered by any settlement short of full independence for a Greater Croatia including all of Bosnia and Herzegovina.

Fearing an invasion by the Axis powers, Yugoslavia signed the Tripartite Pact on 25 March 1941, pledging cooperation with the Axis. Massive anti-Axis demonstrations followed in Belgrade.

On 27 March, the regime of Prince Paul was overthrown by a military coup d’état with British support. The 17-year-old Peter II was declared to be of age and placed in power. General Dusan Simovic became his Prime Minister. The Kingdom of Yugoslavia withdrew its support for the Axis de facto without formally renouncing the Tripartite Pact. Although the new rulers opposed Nazi Germany, they also feared that if German dictator Adolf Hitler attacked Yugoslavia, the United Kingdom was not in any real position to help. Regardless of this, on 6 April 1941, the Axis powers launched the invasion of Yugoslavia and quickly conquered it. The royal family left the country and Prince Paul was interned by the British in Kenya.

The Kingdom of Yugoslavia was soon divided by the Axis into several entities. Germany, Italy, Hungary, and Bulgaria annexed some border areas outright. A Greater Germany was expanded to include most of Slovenia. Italy added the Governorship of Dalmatia, part of Macedonia and Kosovo, Montenegro, southerly part of Croatia, and more than a third of western Slovenia to the Italian Empire. An expanded Croatia was recognized by the Axis as the Independent State of Croatia. On paper, the puppet state was a kingdom and the 4th Duke of Aosta was crowned as King Tomislav II of Croatia.

The rump Serbian territory became a military administration of Germany run by military governors and a Serb civil government led by Milan Nedic. Nedic attempted to gain German recognition of Serbia as a successor state to Yugoslavia and claimed King Peter II as Serbia’s monarch. Hungary occupied several northern regions.

King Peter II, who joined several other European monarchs in London, was still recognized as the King of the whole state of Yugoslavia by the Allies. From 13 May 1941, the largely Serbian “Yugoslav Army of the Fatherland”, popularly dubbed as “Cetniks” resisted the Axis occupation of Yugoslavia. This anti-German and anti-communist resistance movement was commanded by Royalist General Dragoljub Mihailovic. For a long time, the Cetniks were supported by the British, the United States, and the Yugoslav royal government in exile of King Peter II.

However, as the fortunes of war had changed, the Communist guerrilla led by Josip Broz Tito grabbed more and more power. In 1943, Tito proclaimed the creation of the Democratic Federative Yugoslavia. The Allies gradually recognized Tito’s forces as the stronger opposition forces to the German occupation. They began to send most of their aid to Tito’s Partisans, rather than to the Royalist Cetniks. On 16 June 1944, the Tito–Subasic agreement was signed which merged the de facto and the de jure government of Yugoslavia.

In early 1945, after the Germans had been driven out, the Kingdom of Yugoslavia was formally restored on paper. But real political power was held by Tito’s Communist Party. On 29 November, King Peter II was deposed (and the monarchy abolished) by Yugoslavia’s Communist Constituent Assembly while he was still in exile. The new Yugoslavia covered roughly the same territory as the Kingdom had, but it was no longer a unitary monarchy but a federal republic ruled by the Communist Party