Despite neither character having spoken lines in the film to verify where they're from, anyone who reads up on the making of Aliens knows Wierzbowski and Crowe were played by British stuntmen. If we assume that the characters are British, does that mean they could have once been part of an in-universe UKCM? (Could also mean Spunkmeyer was originally UACM, but due to me not paying attention to the AvPGalaxy interview with Daniel Kash, I made Spunkmeyer a New Yorker in my fanfiction.)

If a UKCM existed in-universe, and Wierzbowski and Crowe were attached to a USCM unit, it's possible that both militaries have some sort of agreement where they can mix units for off-world missions, or the characters volunteered to be part of a USCM unit after going through UKCM boot camp, or Wierzbowski and Crowe became US citizens before enlistment.

I'd like to hear the rest of AL's ideas, though. The 501st group eventually entered Star Wars media. It's possible the UKCM, UACM, and the other divisions could become canon/semi-canon.

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Too complex.
They're ex-pats, who left the UK following Brexit.

The UKCM is canon through aliens:colonial marines. I can't remember if UACM was stated in A:CM or not.

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PFC Mike Rizzuto MVM
USS Blackhawk
3rd Batt 12th Regt

As a semi-related side note. I believe Crowe did actually have a line. I had read about the search for actors in how they wanted to find ones that could do an american accent if they weren't American (like William Hope).

In the drop ship when Frost says "I've got a bad feeling about this drop", I do believe it is Crowe that says "man you always say that. You always say I've got a bad feeling about this drop'". Too my ears at least, it sounds like a fake American accent. He lays the drawl on just a hair too thick. In fact, in the video of Tip's interview with Jonathan Ross, if you imagine he put some base in his voice to sound tough instead of speaking normally, I feel pretty sure that is him. Def wasn't the voice of any of the others stars who we have heard speak. And for what it is worth, many online listings of the quote also give Crowe credit for that line.

So basically, I don't believe Crowe was any more UKCM than Gorman was, since they were selling the "American" accents.

However, as stated The UKCM has appeared in video game form (and before all critical panning and lawsuits, it was declared by fox to be canon).
Several prominant UKCM members from this forum even appeared in it..