World War I (WWI) began in Sarajevo with the assassination of the Austrian Archduke by a Serbian nationalist. This act caused Austria-Hungary to react, followed by the defense of Serbia by their allies, the Russians and French; soon after war had begun. WWI devastated Serbia since much of it was fought on Serbia land, but the result was a growing empire. After the war, Serbia united with Montenegro, Bosnia, Slovenia, and Croatia to create what was later known as Yugoslavia, with Serbia acting as the dominant force in the alliance.
In 1941 Yugoslavia was invaded and quickly overcome by Fascist Italy and Nazi Germany. The Croatians joined the Germans, while the Serbs united more closely with Russia, creating hostility between the two countries, which can still be seen today. After the war, Yugoslavia's government was ruled by the communists, but weren't aligned with the Soviet Union.
Yugoslavia was dominated by Serbs prior to WWII, but in the 1950s and 1960s the Croatians and Muslims gained more power and, as rights were extended to Muslims, Serbs were removed from power to accommodate these changes, a move that created much hostility between the Serbs and the Muslims and Croatians.
In 1989 an ethnic Serb, Slobodan Milosevic came to power in Yugoslavia and immediately began removing the rights of the countries within Yugoslavia, including Slovenia, Croatia, Albania, Macedonia, Bosnia, and Kosovo. This led to multiple independence movements from these future countries and the outbreak of war and genocide. Only Montenegro remained tied to Yugoslavia as Bosnia and Kosovo were the focal point of the Serb-led government's attacks. This led to Belgrade becoming a target for United Nation (UN) bombings through 1995.
Since independence Serbia has struggled to regain a positive international image, lost Montenegro in 2006, and is still rebuilding from the war.