Czech and Slovak: languages or dialects?

Radio Prague International - Topic «Czech language course»

“A language is a dialect with an army and a navy.” This is the maxim often cited to explain the sometimes arbitrary distinctions between languages and dialects. So what about the case of Czech and Slovak, which are generally considered to be mutually intelligible? Were they always considered to be separate languages, even when Czechoslovakia was one country? And has anything changed linguistically since the nations went their separate ways politically?