the US had enough power to conquer the whole world in the end of WW2, not just nukes but conventional forces. it had tens of millions of troops, thousands of ships and tens of thousands of airplanes. its industrial capacity could not be matched.
even now US still has the ability.
the thing with US is they were not like territorial animals like mongols etc. they rather trade than outright conquest and wars. why they are like that, uncertain but could be due to christianity. in this religion everyone is supposely equal so a country/people don't really have valid causus belli to declare war on others simply for territory. the US itself fought among themselves for niggers in large part due to religious influence