USA already had its genocidal expansionist moment as a nation before the great World Wars. An entire continent ethnically cleansed and replaced with Anglo settlers. Of course, there was no one to hold it accountable as with the Germans and Japanese. This greatly inspired 20th century fascist thinkers around the world, including Adolf Hitler.

Lebensraum, spazio vitale, manifest destiny.

Even ignoring that, if "nationalism" does not include the massive public projects and social cohesion of 1930s-50s USA, then frankly you have a bizarre definition of nationalism.