while yes, the 13 colonies came from britain, from even then 'til now, the us is still made out of numerous cultures and people - to simply the us as a mere byproduct of the british empire is to ignore the diversity that has and continues to develop in the us. of course, it's true, the us is highly allied w/ the british (altho don't forget, only a few hundred years ago the british were invading and burning down the white house!). but u have the spanish influenced west and southern states, louisiana was once french, not to mention the native americans who lived across the region. the us population today is hardly made up of those w/ english ancestry - even during the nation's founding, the us was still made up of seveal european ethnicties, including german, french, etc.
i mean, if one's reason to exclude to us is b/c it came from britain, then u mite as well take out the koreans and japanese, as they're believed to have migrated from china... or take out some european civs, i mean germany wasn't even a country until after the us was formed. the aztecs came much later in relation to the mayans, and only were around for a few hundred years.
b'sides, taking the world and mankind a button away from nuclear annihilation along w/ the ussr i think hardly makes them a secondary player in world history, not to mention sending people to the moon, fighting world wars, inspiring people to lead revolutions in their own countries, etc.
