An invasion of Greenland, or Canada, wouldn't necessarily be unlawful.
Remember 9/11? Remember George Bush asking Congress to approve his use of military force to hunt down the suspects?
Well that Authorization for the Use of Military Force, unlike any prior which had clearly defined limitations, was simply against "terror" and set to expire "never."
One member of Congress refused to vote for this, precisely because she understood that Congress was effectively forever giving up its ability to determine when and how the President was allowed to deploy the military. She got death threats. She was right.
All Trump has to do is "find" a terrorist threat in a country, and he's allowed to send US troops there. Remember how he recently decided that fentanyl is a weapon of mass destruction? Yeah.
Yeah, there's no question that Putin helped to get Trump elected because he, like any halfway intelligent person, could see the damage he would do.
It's the notion that Trump is checking in weekly asking for new orders that annoys the shit out of me.
Also, I will be stealing "Fire-and-forget idiot", that is fucking excellent.