After the First World War, the Kingdom of Serbia became part of the newly founded "Kingdom of Serbs, Croats and Slovenes", which stretched from the Western Balkans to Central Europe. This territory was ethnically very diverse. Tito, who would later lead the region, famously said: "I am the leader of one country which has two alphabets, three languages, four religions, five nationalities, six republics, surrounded by seven neighbours, a country in which live eight ethnic minorities."
Right from the start, problems arose between the different ethnic groups. In 1929, King Alexander I tried to curb nationalist and separatist tendencies by turning the country into a dictatorship and renamed the country "Yugoslavia". He also decided to abolish the country's historic regions and drew new internal boundaries for provinces, or banovinas, that disregarded all historical and ethnic lines.
Alexander I's plan failed when, in April 1941, Axis troops conquered Yugoslavia; many of its citizens didn't mourn its passing. The country was split up: an independent Croatian state, which also included most of Bosnia and Herzegovina, was created under the rule of the fascist Ustashe movement. They conducted large-scale genocide campaigns against Serbian, Jewish and Roma citizens. The area of (roughly) present-day Serbia and the northern part of Kosovo became the occupied "Territory of the Military Commander in Serbia" and was placed under under German Military administration, with a puppet administration led by Milan Nedić, a former Minister of Army and Navy of the Kingdom of Yugoslavia.
During the Second World War, a civil war broke out in Yugoslavia between the Croatian Ustashe, Serbian pro-monarchist partisans, or "Chetniks," and communist partisans, led by Josip Brosz Tito. After the war, the communist Socialist Federal Republic of Yugoslavia was founded. Tito first became the prime minister and later the longtime president, ruling the country until his death in 1980.
Josip Broz Tito (1892-1980) was the most defining figure of 20th century Yugoslav history. Here you can watch a documentary about Tito. In 1948, after a conflict between Tito and Stalin Yugoslavia was expelled from the international association of socialist states Cominformm. In the following years Yugoslavia developed its own version of communism - Titoism.
Tito was a popular public figure in Yugoslavia, viewed as a unifying symbol for the Yugoslav federation. He is also named the architect of Yugoslavia's disintegration, however, and remains a controversial figure. Read this Time Magazine article on the life of Tito.
The delicate balance between the different ethnic groups in Yugoslavia was disrupted during the 1990s. Since the early 1980s, economic problems and rising nationalism amongst the various ethnic groups had already escalated tensions. In 1990 these tensions led to a series of conflicts. Political upheavals and devastating civil wars erupted, resulting in the dissolution of the Yugoslavia. Here you can find a brief overview of the conflict.