Originally Posted by
Tankfriend
I guess there is, as long as you don't consider this as "cheating".
Simply save your game and then surrender to end the game. Now, go to the campaign selection screen and select your recently made game save, then select the US flag on the top and load the game.
Now you're playing the exact same game, just as the US.
That way, you should be able to go to the diplomacy screen and leave the alliance on your own accord.
Save, surrender and change to your original faction again and you've gotten rid of the US as a member of your alliance.
If that's not possible as well, you should try to end the war first as EvilManicafromMars has implied above.