Czechoslovakism (Czech: Čechoslovakismus, Slovak: Čechoslovakizmus) or Czechoslovak nationalism, is the nationalism of Czechoslovaks and Czechoslovak culture either for which Czechs and Slovaks embrace a Pan-Slavic state in which they function as constituent nations (political form), or for which the two nations form a single West Slavic ethnic group (ethnic form of Czechoslovakism). Czechoslovakism is based on natural cultural and language affinity of Czechs and Slovaks and has roots in early romantic national movements (works of Ján Kollár or František Palacký). The idea had been continuously developed and supported by common meetings and organizations like Czech-Slavic Unity (Českoslovanská jednota), where Czech sympathies for Slovaks and their spontaneous reactions on oppression of Slovaks in the Kingdom of Hungary played a partial role. However, both nations also developed other alternatives like Austro-Slavism, close collaboration with other non-Hungarian nations in the Kingdom of Hungary or pan-Slavic orientation on Russia. Before World War I, Czechoslovakism was further developed by Tomáš Garrigue Masaryk and Edvard Beneš, based on the premise that Czechs and Slovaks are one people who had become divided due to Austro-Hungarian imperial rule and Hungarian assimilationist policies in particular. These Czechoslovakists noted that a united Czechoslovak people existed in the historical state of Great Moravia, where the Slavonic tribes of Bohemia, Moravia, and Slovakia were formerly embraced within Great Moravia. Czechoslovakists typically emphasize secularism to break down the religious divide between Czechs and Slovaks.
While the Cleveland Agreement in 1915 and the Pittsburgh Agreement on May 31, 1918 made allowances for both a Czech and a Slovak nation, the Washington Czechoslovak declaration of independence on October 18, 1918 mentions only the one Czechoslovak nation. The published version of the Slovak Martin Declaration (October 30, 1918) which legitimized Czech-Slovak activities abroad, requested fulfillment of the right to self-determination of the Czech-Slovak nation and the creation of the common state. It mentioned not only the "Slovak nation" and the "Czech-Slovak nation" but also "the Slovak branch of the unified Czecho-Slovak nation". This terminology was compliant with contemporary needs and views since not all of the delegates agreed with ethnic Czechoslovakism. However, they supported the idea of a single state and aligned their formulations with foreign Czech-Slovak organizations. The original approved text of the Martin Declaration was modified after the return of Milan Hodža from Budapest. The change consisted in removing the demand for a separate Slovak delegation in the peace conference and the reference to the latest diplomatic note of the Austro-Hungarian Foreign Minister recognizing the Fourteen Points of Woodrow Wilson. This later raised various discussions and some participants claimed that the published text was a forgery (proceedings were temporarily lost but are available to modern historians).
This ideology was essential to the creation of Czechoslovakia in 1918, and for the establishment of the Czechoslovak Constitution of 1920. Statistics from the era of the First Republic (1918–1938) mentioned Czechoslovaks rather than Czechs or Slovaks:
Nationalities of Czechoslovakia 1921
During the inter-war period, ethnic or political Czechoslovakism was supported both by Czech and Slovak representatives. However, while Czechs perceived Czechoslovakism as an expansion of, rather than a threat to, their original Czech identity and did not develop any real opposition to the idea of ethnic unity, in Slovakia the idea was strongly criticized by the autonomist movement.
During World War II, when Czechoslovakia was occupied by the German Third Reich, the Slovak Republic was created as a client state of Nazi Germany, and Subcarpathian Ruthenia was annexed by Hungary. Slovak democratic and communist resistance movements preserved the idea of political Czechoslovakism and agreed on restoration of the common state. During Slovak National Uprising the rebelling Slovak Army declared itself to be a Czechoslovak army and restored Czechoslovak rule in the controlled territory. After World War II, Czechoslovakia was reunited, but the ideology of one ethnic nation was not fully restored.
After the 1960 Constitution of Czechoslovakia had limited the autonomy granted to Slovakia, the 1968 Constitutional Law of Federation stated that Czechoslovakia was a federation of two national republics and introduced dual citizenship.