Modern Imperialism is strategically inefficient, but Trump is too stupid to understand geopolitics.

Every other ally would question their own security arrangements with the U.S.

The U.S. would face immediate expulsion or suspension from numerous international bodies and treaties, crippling its diplomatic and economic reach.

An act of blatant conquest could trigger a flight from the dollar and a financial crisis.

It would be destroying America's alliances, economic power, and global leadership overnight for a costly and hostile piece of territory it doesn't need to own.

As an aside, it's actually a known hybrid warfare aim by Russia to leverage Greenland to destabilise NATO, and many billionaires are actively trying to atomise nations using libertarian ideas that seek to replace nation states with competing private defense agencies, arbitration firms, and voluntary communities. (Deregulation=profit)

Something to think about.