Nonsense. That or your definition of “enemy” is so broad that it includes allies of a different culture and different constitution. For example…Americans are nationalist concerning the Brits. We like em. They are allies. We share much culture. But we don’t want to be British and vice versa. And if we get into trade disputes, all else being equal we want American concerns to prevail. That’s “America First”. If they were ab actual enemy we would not only want our interests to prevail but would possibly go further to damage the enemy’s interests.
Like Communism, it is possible on a theoretical level that a “pure” nationalism could exist that did not require an enemy to focus the will of the people on - in the same sense that theoretically, we could all put aside all our animosity and work side by side for the betterment of all humanity.
But nationalism has never worked without an enemy.
Well…
China is an enemy. But Canada is not and yet American Nationalism is relevant in both cases. We can’t have Canada ruining our industries that are needed for national security. So “America First” principles are relevant for our relationships with both countries.