A lot of war movies that depicted Americans in world war 2 has limited communications and relations with international allies and that got me wondering.
One scene in particular from band of brothers where Americans drove by soviet soldiers that were executing German soldiers; Americans just kept driving like they were strangers. Got me thinking a bit.
Well, the scene in which you are referring to was showing Americans driving by French soldiers committing war crimes.
In this case the movies were rather accurate, as it was incredibly unusual to serve alongside other nationalities. Troops were organized into units for their own nationality - besides all of the issues of equipment/supply/training/command, it is simply easier if they all speak the same language. Some people on the internet complain whenever an American movie/videogame shows up about "WHERE ARE ALL THE CANADIANS/BRITS/FRENCH?" but in reality an American unit would be made up of Americans, with international differences really just showing up in the ethnic makeup of the Americans themselves (Italian/Irish/Whatever immigrants).
For example, while the D-Day landings featured many different nationalities, the Omaha Beach assault depicted in Saving Private Ryan (and thus every other videogame with D-Day) was carried out by American forces. There were 5 invasion beaches, of which 2 of them were American, and as Omaha was the bloodiest it was the most famous. Utah, the other American beach, was considered something of a cakewalk so we never see it in movies.
There were exceptions of course, especially in the case of resistance groups, special operations, and that sort of thing, but these would always be rather small cases.
It was actually more common among the Germans to have an international flavor to their units, as they frequently got hundreds of thousands of volunteers/conscripts from different countries. These were often put into units commanded by German officers.
The Soviet Union also featured many different nationalities in its army, so a Russian soldier was more likely to be serving near a Mongol or an Uzbek, for example.
On many occasions as US forces built up in Britain before the invasion, violence broke out between British and American soldiers, as White American soldiers tried to enforce segregation in pubs and clubs. The British, with no tradition of segregation and a natural inclination to side with the underdog, invariably took the side of the Black American soldiers, and large brawls took place, with British soldiers and civilians alike running the Americans out of town whenever they tried to evict a black soldier from the bar.
There were other tensions too, with the British saying of the Americans "They're overpaid, over-sexed, and over here". While this was in the main benign banter, the wages of British soldiers were lower than those of Black Americans, which were in turn lower than Whites. In addition, Americans could get hold of items the British couldn't as easily, such as nylons, chocolate and tobacco. This made them very popular with the British women.