At the outbreak of World War I, the Czechs and Slovaks showed little enthusiasm for fighting for their respective enemies, the Germans and the Hungarians, against fellow Slavs, the Russians and the Serbs. Large numbers of Czechs and Slovaks defected on the Russian front and formed the Czechoslovak Legion. Masaryk went to western Europe and began propagating the idea that the Austro-Hungarian Empire should be dismembered and that Czechoslovakia should be an independent state. In 1916, together with Eduard Benes and Milan Stefanik (a Slovak war hero), Masaryk created the Czechoslovak National Council. Masaryk in the United States and Benes in France and Britain worked tirelessly to gain Allied recognition. When secret talks between the Allies and Austrian emperor Charles I (1916-18) collapsed, the Allies recognized, in the summer of 1918, the Czechoslovak National Council as the supreme organ of a future Czechoslovak government.

In early October 1918, Germany and Austria proposed peace negotiations. On October 18, while in the United States, Masaryk issued a declaration of Czechoslovak independence. Masaryk insisted that the new Czechoslovak state include the historic Bohemian Kingdom, containing the German-populated Sudetenland. On October 21, however, German deputies from the Sudetenland joined other German and Austrian deputies in the Austrian parliament in declaring an independent German-Austrian state. Following the abdication of Emperor Charles on November 11, Czech troops occupied the Sudetenland.

Hungary withdrew from the Hapsburg Empire on November 1. The new liberal-democratic government of Hungary under Count Michael Karolyi attempted to retain Slovakia. With Allied approval, the Czechs occupied Slovakia, and the Hungarians were forced to withdraw. The Czechs and Allies agreed on the Danube and Ipel' rivers as the boundary between Hungary and Slovakia; a large Hungarian minority, occupying the fertile plain of the Danube, would be included in the new state (see fig. 6).

Data as of August 1987