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, HRH Regent Alexander Karadjordjevic proclaimed the Kingdom of the Serbs, Croats, and Slovenes, under HM 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, but also because of the provocation made when it was said in the Parliament: “How much does your Serbian blood cost? How much does Kajmakčalan cost? Let’s pay – and then we can be at peace!” 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 rule. The King did this against his will, promising to restore democracy in the renamed Kingdom of Yugoslavia once national unity was achieved and corruption in the state apparatus was eradicated.

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 these events occurred, 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 late 1934, the King planned to release Macek from prison, introduce democratic reforms, and attempt to find common ground between Serbs and Croats.

Alexander I was among the first statesmen in Europe to see the danger posed by the rise of extreme ideologies – Nazism and Fascism. He fell as their victim, being assassinated in Marseille on 9 October 1934, 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, King 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 King Peter II was declared to be of age and placed in power. General Dusan Simovic, one of the organisers of the 27 March Coup, 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. With the invasion of the Kingdom of Yugoslavia by the Nazis and their allies, King Peter II, as well as the Yugoslav Government, was forced to leave his homeland and, via Athens, Jerusalem, and Cairo, join other heads of state from occupied Europe in London. 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 other European monarchs in London, was still recognized as the King of the whole state of Yugoslavia by the Allies. King Peter II made a huge effort to help the people of Yugoslavia in the fight against Nazism, and he also constantly tried to return to the country and be with his people. He sought support for his countrymen, meeting with many important world leaders of the time, including President Roosevelt, Sir Winston Churchill, his godfather King George VI of the United Kingdom, Queen Wilhelmina of the Netherlands, President of Czechoslovakia, Edvard Benes, King Haakon of Norway and Mr Raczkiewicz, President of Poland. He was the youngest head of state ever to address the United States Congress, in 1942.

Despite the collapse of the Yugoslav Army, two rival resistance movements emerged in the country. The first was the Yugoslav Army in the Homeland, a legal and legitimate military formation led by General Dragoljub (Draža) Mihailović, Minister of Defence of the Government in Exile, whose goal was to liberate the country and preserve the pre-war constitutional order. The second was the revolutionary partisan movement, led by the communist Josip Broz Tito. In addition to the conflict with the occupying forces, a bloody civil war broke out in Yugoslavia between the two movements.

The Allies initially supported Mihailović, exhibitions were organized about him and his fighters, and 20th Century Fox even made a film in 1943 called “Chetniks, the Fighting Guerillas”. However, under the strong influence of Soviet spies who infiltrated the British Special Operations Unit (SOE) and their propaganda, before all James Klugmann who spent two and a half years working on the headquarters staff of the Yugoslav Section of SOE, they reoriented towards Broz. Also, individuals such as Randolph Churchill, son of British Prime Minister Winston, and Fitzroy MacLaine, who served as officers in the communist headquarters, had a great influence on changing the Allied attitude towards the Yugoslav Army in the Homeland, as well as the United Kingdom’s representative to the Allies in the Mediterranean, Harold Macmillan, who insisted that both Great Britain and the Yugoslav government and King support Broz and the communists “with all their powers”. The pressure went so far that King Peter II, who despite this development of the situation insisted on moving to Yugoslavia to join Mihailović, received a warning from the British side that his policy of supporting Mihailović would provoke a civil war and lead the Red Army to physically destroy all the troops of the Yugoslav Army in the Homeland.

Despite this development of the situation, General Mihailović and his soldiers continued the unequal fight, organizing one of the largest rescue operations of Allied pilots shot down behind enemy lines – Operation Halyard. For these merits, General Mihailović was posthumously awarded the Legion of Merit, First Class, by US President Harry Truman on 29 March 1948. On the other side, in 1944, with the help of the Red Army, the partisans entered Belgrade and formed a communist government.

On 29 November 1945 the monarchy was illegally abolished in undemocratic elections and Yugoslavia became a totalitarian one-party state under the rule of the Communist Party.