One of my favorites! Must have seen it at least a dozen times. I hear that Bogart wasn’t fond of it.
Also just recently saw Kanal, on Netflix DVD. I suggest watching before before it another Polish film “A Generation” also relating to underground activities during the German occupation and sort of sets the stage for Kanal.
There is an excellent Polish TV series called "Time of Honor" (Czas Honoru) about the Polish Underground during WWII.
I’ve actually seen A Generation as well. Excellent film.
BTW, I dunno if you knew this, but there’s a third film in the series. Ashes and Diamonds came out in 1958, and IMHO, it’s the best of the three movies. It’s very bleak, but an excellent film. The lead actor (Zbigniew Cybulski) was known as “The Polish James Dean.”