The First Yugoslavia
Main article: Kingdom of Yugoslavia
The goals of the Yugoslav Committee were partly reached by the end of the First World War in 1918, when Austria-Hungary disintegrated, and the South Slavs organized into the State of Slovenes, Croats and Serbs. This short-lived state soon, on December 1, 1918, joined Serbia and Montenegro to form "The Kingdom of Serbs, Croats and Slovenes". In the chaotic conditions that followed the break-up of Austria-Hungary, the formation of the new state as soon as possible was a priority. Disagreements over whether the new state should be a federation or a centralised state were put off for later.
On June 28, 1921, — a day of historical importance to Serbs (see Vidovdan) — parliament (Skupština) passed a new constitution despite a boycott from Croatian MPs. The constitution centralized political authority and strengthened the power of the royal government in Belgrade, causing dissatisfaction among the more federally minded Croat and Slovene politicians.
In 1928, Puniša Račić, an ethnic Serbian nationalist leader from Montenegro, shot and killed Croatian Peasant Party Leader Stjepan Radić in the parliament chambers. King Alexander (Aleksandar) used the shooting as a pretext to strengthen his power and on January 6, 1929 he suspended the constitution, dissolved the Skupština and proclaimed a royal dictatorship. He went on to reorganize the regional divisions within the country and renamed it the Kingdom of Yugoslavia. All national identities except "Yugoslav" were abolished.
Yugoslavia became a highly militarized state, which spawned several insurgent nationalist groups opposed to the royal dictatorship. The king was highly unpopular, particularly among non-Serbs, and while on a visit to Marseille, France in 1934, he was assassinated by Bulgarian nationalist and IMRO activist Velichko Kerin (more popular with his revolutionary pseudonym Vlado Chernosemski). His son and successor, Peter II (Petar II), was a child, so power fell into the hands of the ineffectual Prince Paul (Pavle), who continued on an authoritarian path with the prime minister Milan Stojadinović.
In the beginning of World War II, Yugoslavia was pressured by Germany and Italy to join the Axis powers. Italy was mired in an inconclusive war with Greece, and before Germany committed its forces to the Greek campaign, it wanted to secure Yugoslavia's support.
Prince Paul submitted to the fascist pressure and signed the Tripartite Treaty in Vienna on March 25, 1941, hoping to still keep Yugoslavia out of the war. But this was at the expense of popular support for Paul's regency. Senior military officers were also opposed to the treaty and launched a coup d'état when the king returned on March 27. Army General Dušan Simović seized power, arrested the Vienna delegation, exiled Paul, and ended the regency, giving 17 year old King Peter full powers.
Hitler then decided to attack Yugoslavia on April 6, followed immediately by an invasion of Greece where Mussolini had previously been repelled. (As a result, the launch of Operation Barbarossa was delayed by four weeks, which proved to be a costly decision.)

The First Yugoslavia
Yugoslavia during the Second World War

 The Second Yugoslavia
Breakup
Federal Republic of Yugoslavia