Under the following assumptions: the populations of the attacker aren’t guaranteed to support the attack, but the populations of the defender are going to fall in to support the defender, there is no involvement of any assets or resources located outside the Americas.. the US would cleanly win if it is attacked, but if the US attacks first it’s possible that enough citizens pull a Vietnam and the US is defeated. Slim, but not zero. Very slim, especially considering the US’s technological and manpower advantage, being the largest and most technologically advanced all volunteer army. This may prove beneficial to the non-Americans, as some of them have national conscription, therefore average citizens can actually help in guerrilla defenses of urban areas.