Well, the Slovenes and Croats were also part of an multinational state, Yugoslavia, which formed concurrently with Czechoslovakia at the end of the First World War (originally as Kingdom of Serbs, Croats and Slovenes). But while Yugoslavia understood itself from the outset to be a merger of peoples, the most influential founders of Czechoslovakia perceived the Czechs and Slovaks to be one people that had been separated by outside powers for a few centuries.

This perception has its roots in Great Moravia, an empire that existed in Central Europe for about seven decades during the 9th and early 10th centuries. Both the Czechs and the Slovaks perceived their nationality as descending from this empire. The key here is that the Cyrillic writing system was introduced to Great Moravia by Saints Cyril and Methodius during the first half of the empire's existence. The twentieth-century Czech and Slovak languages both descendent from the standardized written language of Great Moravia, and in fact Czech and Slovak remained highly mutually intelligible throughout the centuries. After the fall of Great Moravia, however, a distinct Czech and Slovak culture formed as the two ethnicities were passed from one power to another, never under the same state again, until the Hapsburg empire absorbed the Hungarian empire, which had control of the Czech and Slovak populations, respectively. Even while under the Hapsburgs and later the Austro-Hungarian empire, however, the two peoples continued to have very different influences, as Prague, the Czech capital, became a German city at the center of Hapsburg industry, while the Slovaks remained an agrarian people whom the Hungarians tried to fully absorb into Hungarian culture in a process known as "Magyarization" (a sore spot for nationalistic Slovaks to this day).

When World War I began, several influential figures in Prague began planning for full Czech independence, and several of these plans included a Czecho-Slovak state. Thomas Masaryk, whose pro-Western movement beat out the pro-Russian movements and led Czechoslovakia to independence, did not perceive this as a matter of convenience, however. Masaryk truly believed in the single Czechoslovak identity, because he himself was a true "Czechoslovak." Masaryk was born in a part of Moravia (a Czech land) in what was official under the administration of Hungarian Slovakia. His father was a Slovak and his mother Czech. You combine this with the mutual intelligibility of the two languages, and you find Masaryk believing that the Czechs and Slovaks were one people who were to be reunited in independence. His vision carried the day, and the "Czechoslovak experiment" began in 1918, when he declared independence from Washington, DC in a declaration officially distributed from Paris.

As it would turn out, of course, the differences between the Czechs and Slovaks were not as inconsequential as Masaryk had envisioned, and Slovak nationalists began calling for greater autonomy for Slovakia less than a decade after the nation's creation.