President-elect Donald Trump says he won't rule out the use of military force to seize control of the Panama Canal and Greenland as he declared U.S. control of both to be vital to American national security.
It likely has a lot of mineral wealth. It's best to get control of that wealth, before some dirty locals try to setup their own government with rules about respecting them, their culture or property.
At least Greenland, being part of Denmark, is also a part of NATO, which basically rules out military force. Unless you literally want to start another world war. Don't do it, Donnie.
I believe the "rationale" (for lack of a better word) is that the land of Greenland will become much more valuable in a post-climate-change world. Much of greenland will become warmer and therefore more comfortable to live in and more suitable for farming. So the idea is to take it now while it's still largely a chunk of uninhabitable ice rather than wait until later when everybody else is fighting for it too.
With Greenland, he wants control of the Arctic. China and Russia are also competing for control. But this is just the wrong way to go about it in every sense of the word.