Like everything, one should not oversimplify
That bit of Czechia was Cieszyn
For centuries, this was a mix of Czechs and Poles (both are West Slavs and their languages are somewhat mutually intelligible - kind of like the French and the Franco-Swiss
Anyway, at the end of WWI, the German, Austrian and Russian empires collapsed and everywhere was a free-for-all
Cieszyn was a mix of Poles and Czechs -- it is also one of the only 3 areas where an army can easily march across the Carpathian range, so very important strategically (it's also the largest such pass and the easiest) and yet the local administrators of Cieszyn came to a good agreement of how to split this -- they did so amicably. But this was not satisfactory to the politicians in Prague or Warsaw -- Cieszyn was and is rich in mining items. And the Western nations arbitrarily divided the land, giving the majority of the land to the Czechs even the places which were 90% Polish
The Poles and Czechs had skirmishes over this but to no avail until 1938.
The Poles wanted to deny the Germans the Cieszyn pass and they took itwhen the Germans took most of the Czech Republics mountain area
Was it right to do this? Morally ambiguous (it was polish majority yet this was participating in the slicing up of another country) but strategically good (though it didn't help them in the end
Once the Nazis took the Sudetenland, Czechoslovakia, for all intents and purposes ceased to exist as a viable state. The Nazis only made if official a few months later.