Is stalag 17 about US immigration, how people come from different countries to become part of a united America? So is being American an ideology or political commitment and who is left out of the dream in Stalag 17
I would say that being American is an ideology that encompasses political commitment. The as mentioned, part of the American ideology is that we are an open country which accepts others. That's how the nickname "melting pot" was coined. That being said, our country must then embrace specific political ideology to make the American idea work.
I don't think that the movie is about that, but it certainly (like a lot of movies from that time period) emphasizes that the US is this melting pot. You tend to see this a lot in WWII movies because it emphasizes how the US is not racist like the Nazis. By the '50s, it also made sense to show how inclusive the US was because it was a way of showing we were commited to democracy, unlike the communists.
The group that is most clearly being left out in this movie is anyone non-white. You have Jews and Italians and Poles and such, but you have no one with a Spanish name and no one who is black or Asian. Of course, these were all airmen and minorities were mostly kept out of that branch.
Ideologically, the only people who are left out are those who will not accomodate themselves -- those who will not act like everyone else in order to get along.