And the US in turn abdicated its separation of powers. A US president lacks the ability to make treaties, or use military force without congressional authorization.
In what world does the president have the authority to annex an autonomous territory from an ally?
The world where he does it and then tries to present it to Congress as a fait accompli. If the security concerns around Greenland are seen as legitimate enough, he’ll get his Democratic congressmen to approve it, particularly as it’s unlikely that it would become a state (too few people).
In what world does the president have the authority to annex an autonomous territory from an ally?