Yeah! Americans are God's chosen instruments of vengeance and rescue! And we provide transportation to Elfs, Dwarves, and Hobbits in need all the time...
No, I don't think that the Eagles represent anything except eagles, any more than I think Dwarves represent anything but Dwarves, or that Isengard means anything. Tolkien's not the symbolism type at all, I'd say. But it's an interesting idea...
Well, the only reason I thought they could be Americans was because of when they came into the battles. When all looked lost... then suddenly "The Eagles are coming." And if you lived during World War I and World War II, then both times, when the war looked lost, the Americans came.
Of course, I don't think the books are allegories... I just think they are great.