unlike the US who feels obligated to accept an unlimited number of Earths population.

Population. Boom.

The USA has been historically attractive as the 'land of immigrants', 'land of opportunity.' Change that and we become just one more country looking out for itself; at odds with all the rest of the nations. Americans will no longer be able to travel the world with the relative impunity we enjoyed.

So what's different now? Jets. Satellites. Nuclear subs. Missiles. Depleted marine protein. Rising seas. Droughts. Pollution.

And, dare I say it.... human overpopulation competing with robotics.

AI feeding the masses with information.

If the USA becomes isolationist, puts major military power on the southern border, 1) we will no longer be that 'shining beacon on a hill.' 2) Nations seeking expand (China, Russia, Korea) will do so with impunity, and
3) Israel will be wiped out and God only knows what will happen to the remainder of the the Jewish diaspora as Christian nationalists.

Bigotry, misogyny, racism will increase.

Maybe it's already cooked into the overpopulation pie.

Whiskey's for drinking, water;s for fightin' over.

It was fun while it lasted.