I think some people have been unduly unfair on this film.
There is quite a complex sequence of flashbacks. But as a matter of fact, I didn't find them at all difficult to follow. My brain only hurts when I try to work it out afterwards. Maybe it's another of those things which work better in a cinema than on TV.
There is a scene where Bogart's character commits a war crime. I think we have to remember that Bogart did not always play saintly characters. He was not exactly saintly in the "Maltese Falcon" or "Casablanca". He was even less saintly in "The Caine Mutiny". I am sure that the audience in 1944 would have been shocked by the war crime just as we are
now; even Nazi propaganda sometimes emphasised the importance of being gentlemanly to prisoners. The easy and boring option would have been for
Bogart to play the all-American (or all-French) hero throughout; I find it
more interesting that in this case he isn't. I think the circumstances to some extent explain what Bogart's character does. The fact is war crimes happen in war. They happened then, and they happen now, and the perpetrators are not as through-and-through evil (or different from us) as we would like to think.
I agree with those who say this film is not as good as "Casablanca" or the "Maltese Falcon". The plot is a lot more lumpy and uneven than those films. But I've seen those two films several times already, and I can't watch them every night. "Passage to Marseille" is worth at least one viewing. In fact I would like to see it again, if I get a chance.