Wikipedia (excerpts)
The creation of Czechoslovakia in 1918 was the culmination of a struggle for ethnic identity and self-determination that had simmered within the multi-national empire ruled by the Austrian Habsburg family in the 19th century. The Czechs had lived primarily in Bohemia since the 6th century, and German immigrants had settled the Bohemian periphery since the 13th century. After 1526, Bohemia came under the control of the House of Habsburg as their scions first became the elected rulers of Bohemia, then the hereditary rulers of the country. Following the Battle of White Mountain in 1620, the Kingdom of Bohemia was gradually integrated into the Habsburg monarchy as one of its three principal parts, alongside the Archduchy of Austria and the Kingdom of Hungary. With the rise of nationalist political and cultural movements in the Czech lands (the Czech National Revival) and the Slovak lands (the Slovak National Revival instigated by Ľudovít Štúr), mounting ethnic tensions combined with repressive religious and ethnic policies (such as the forced Magyarization of Slovaks) pushed the cohesion of the multi-national Austro-Hungarian Empire ruled by the Habsburgs to the breaking point.[2]
Subject peoples all over the Austro-Hungarian empire wanted to be free from the rule of the old aristocracy and the imperial family. This frustration was partly eased by the introduction of local ethnic representation and language rights, however the First World War put a stop to these reform efforts and ultimately caused the internal collapse of the Austro-Hungarian empire and the liberation of subject peoples such as the Czechs and Slovaks.[1]

In 1916, during World War I, Tomáš Masaryk created the Czechoslovak National Council together with Edvard Beneš and Milan Štefánik (a Slovak astronomer and war hero). Masaryk in the United States, Štefánik in France, and Beneš in France and Britain worked tirelessly to secure Allied recognition. About 1.4 million Czech soldiers fought in World War I, 150,000 of which died.
More than 90,000 Czech and Slovak volunteers formed the Czechoslovak Legions in Russia, France and Italy, where they fought against the Central Powers and later with White Russian forces against Bolshevik troops.[5] At times they controlled much of the Trans-Siberian railway, and they were indirectly involved in the shooting of the Russian tsar and his family in 1918. Their goal was to win the support of the Allies for the independence of Czechoslovakia. They succeeded on all counts. When secret talks between the Allies and Austrian emperor Charles I (r. 1916–18) collapsed, the Allies recognized, in the summer of 1918, the Czechoslovak National Council would be the kernel of the future Czechoslovak government.

The independence of Czechoslovakia was officially proclaimed in Prague on October 28, 1918[6]