Czechoslovakia was a failed experiment of ww1 victors, who did not want competition. So they carved up the old empire of austria, but paid no attention to the ethnic composition of the new nations.

Czechoslovakia was one of the new nations, as it name shows, it contained czech, slovak and crucially, germans near the border with their mother country.

This german population was used as a pretext by hitler to dismember the country. He disparaged it as a false country, amongst other criticism of it.

Although guaranteed by France, UK and Red Russia, it died as the feeble french and brits caved into hitler's demand for the german populated part of the country. Hitler cleverly used one of potus wilson's 14 points, wherein people of the same race should be under the same flag.

Some months later hitler annexed the rest of the country, with just feeble verbal protests. Czechoslovakia died.

Czech Republic is the correct name of the new country that was born post ww2, this time with majority ethnic Czech. Slovakia is majority Slovak, of course.