Czechoslovakia was formed in 1918, as Austria-Hungary collapsed at the end of World War I.
The new state was formally created in Prague on 28 October 1918. Two days later, Slovak representatives in Martin declared their adherence to the new country. The state united the Czech lands—Bohemia, Moravia and part of Silesia—with Slovakia and, initially, Carpathian Ruthenia.
Tomáš Garrigue Masaryk became its first president, while Edvard Beneš served as a leading diplomat and later president. The country was initially known as the Czecho-Slovak state; the official name was formalized in 1920.
A common mix-up is confusing its formation with its dissolution. Czechoslovakia peacefully split into the Czech Republic and Slovakia on 1 January 1993, in an event often called the Velvet Divorce.