I think - first and foremost - we have to accept that cinema is entertainment and probably will not be 100% accurate because of that. I think it's okay to a certain extent to apply artistic licence but the audience will probably be hostile towards a movie if it's more or less a downright lie (unless, of course, it's a deliberate parody - something like a Mel Brookes movie).
U571 - on the other hand - is just an insult to the real men who obtained the Enigma code machine.
I think, ultimately, writers and directors simply need to know just how far they can go and when the audience will just simply say no.
Apparently the writer, David Ayer, of U571 did meet one of the real men who recovered an enigma machine and apparently the veteran wasn't offended with the script/story,
but Ayer does say now that he regrets distorting history in such a manner and he would not do it again. (After the film got made and he made his fee of course...)
There does seem to be a great commercial pressure particularly for WW2 films made by US studios for them to be Americanised, 'cause that's the biggest market. I think though there is a lot to be worried about such shenanigans, but then again such pressures are as old as, well, any media.
Take the D-day landings. When I was growing up the seminal film about this event was
The Longest Day, a faux documentary, and it looked at all sides and had a reasonably balanced showing of US, French, German and 'British' involvement (I'll come back to why I've marked out us Brits...). Post 1998 the seminal D-day film has become
Saving Private Ryan, and although it's purpose is not to tell the story of the full landings, I think there is a danger that because it is so powerful future generations will make it synonymous with D-day - and perhaps come away thinking that the US did everything. Of course not so, the British
Empire supplied the majority of the initial forces for the landings.
Which leads me to why I can't really have a go at the 'yanks' for this. When the UK film industry churned out war films in the late 40's and fifties you'd be hard pressed I think to find the contribution of the Canadian, ANZAC or South African nations in their films - the big market for the films of course being the UK, so it's all stiff-upper-lipped Brits being brave. Going back to D-day I believe about a quarter of those landed on the beaches on the first day were Canadian and I'm not sure even the
Longest Day portrays this reality. And talking about the Enigma code machine does the film 2001
Enigma film downplay the contribution of the Polish?
Finally I have yet to see a UK war film that shows us the contribution of the 2.5 million Indian army that volunteered to fight for Britain. And add to that the approximately one million black African troops raised by Britain to fight, or the Caribbean men that fly and fought for us as well. (Apologies to any far-flung parts of the old British Empire that I haven't mentioned!)
I suppose films will always be looking at what generates the dollars, or pounds, or euros....