In the aftermath of World War I, the former Balkan Peninsula territories of the Austro-Hungarian Empire came together to form the Kingdom of Yugoslavia.
The new kingdom, for the first few years of its existence, was called the Kingdom of Serbs, Croats and Slovenes. The new nation was made up of the former states of Slovenia, Croatia, Bosnia & Herzegovina, Serbia and Montenegro. The Serbian House of Karadordevic became the ruling house of the new kingdom.
The first definitive postage stamps, for use throughout the kingdom, appeared in 1921. These new definitive stamps featured the portraits of Crown Prince Alexander and King Peter I.
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