What is the unprecedented significance of the movie Casablanca?

The thing about art, generally, is that you can regard it however you like. If you're looking for what was intended, on the other hand, you are vastly more constrained.

It certainly was a political film: The unproduced play "Everybody Comes To Rick's" had been floating around Hollywood for a year when the Japanese bombed Pearl Harbor. The studios were very much concerned about presenting a pro-America viewpoint.

Remember, the studio founders were immigrants and Jews (Louis B. Mayer, and the Brothers Warner were all Russians, Carl Laemmle was German), and Washington D.C. was fond of making noise about regulation and censorship, and getting behind the war effort made good political sense. The country was outraged and ready for war, so getting behind the war effort made good business sense. Never mind that these were people with a good reason for hating Hitler. All this quite apart from any other feelings of patriotism Hollywood people had.

When the Japanese attacked, the studios went all out looking for propaganda, and "Everybody Comes To Rick's" got picked up.

So, if we take this as context let's look at your thesis:

a political film that shows anti-fascist feelings overcome love

OK, it definitely was political.

Now, was it anti-fascist? Maybe, but weakly so. Attitudes toward fascism, socialism and communism were so different back then, I cannot possibly document and explain it all here. The very reading list would be more than I would dare put in a post.

"Casablanca" was definitely anti-Nazi, though.

Was it about larger responsibilities overcoming love's impulses? That's the truest aspect of your thesis, I would say, and a very, very, VERY common theme in the movies of the era. Just off the bat, I'm thinking of Bette Davis in Now Voyager from the same year, who sacrifices her romantic feelings for the man who basically saved her in service of giving his daughter (with another woman, his wife) a better shot in life.

As much as Hollywood loved a frothy romcom where Love Conquered All, they saw that dramatic potential in having that One Great Love set aside for The Greater Good. Dynamite stuff! Boffo box office! A three-hanky picture!

In summary, I would rephrase your thesis as:

Casablanca is "a political film that shows duty must sometimes overcome love".

All summed up neatly by Rick's last little speech:

Ilsa: When I said I would never leave you.

Rick: And you never will. But I've got a job to do, too. Where I'm going, you can't follow. What I've got to do, you can't be any part of. Ilsa, I'm no good at being noble, but it doesn't take much to see that the problems of three little people don't amount to a hill of beans in this crazy world. Someday you'll understand that.

I hope this was of interest.