With Posse Comitatus, the military can’t police things at home, but is there a reverse legal version of Posse Comitatus, whereby the U.S. government can’t use, say, NYPD SWAT personnel in a counterterrorism operation in the Middle East?
(As for why the government would do that, that would be fighting the hypothetical, but maybe these NYPD SWATsters have specific expertise for a particular mission that the SEALs don’t, etc.)