I'd love to play against America, but it would probably sell more if I were to play as a fictional enemy. Aliens, maybe, because they'd obviously be better equipped.
In fact, why not go all out and take on the whole world as the aliens? Oh wait, sorry Destroy All Humans, I didn't see you there.
*Ahem*
Yes, playing against America could be interesting, although some games have done it before; Red Alert comes to mind, the Soviet army was my favourite... until Yuri. U.F.O.s! Yey! But an FPS would be great to see, especially if it's done in a true black ops fashion where you do absolutely nothing to draw attention to yourself, have "security blankets" and can only ever contact your organization in secretive ways.
In fact, for all you know, there could be British soldiers killing Americans right now, the latest terrorist attack could just be a political bait-and-switch by the so-called "good guys", the president or prime minister could be killed tomorrow by their own countrymen and made to look like a terrorist plot, it's all smoke and mirrors that's the beauty of deniable operations. It's nothing more than moving pieces across a checkerboard, it doesn't matter who the pawns are and who's the queen; the only real question is who's doing the pushing?
There's a great game in there, even though this sounds like the ramblings of a conspiracy theorist, the fact that it's plausible, even in fiction, is very intriguing.