I think Trump is trying to make a political statement. As a protege of Putin, his philosophy of international politics is a post-Westphalian "might makes right" type world order. The Kalaallit Nunaat (Greenland) case in particular, is a taunt. The island is a semi-autonomous territory of Denmark, with ambitions for full independence. To someone like Trump, independence is not a status under international law, but the ability to hold one's own against rivals. If Trump were to annex Kalallit Nunaat, what are its 58,000 inhabitants (or even Denmark) going to do, fight back? He's trying to beclown the system of sovereign nation states, which includes a lot of small and/or weak ones that used to be part of empires, as something only fools or idealists believe in. At least Putin is picking on someone 1/4 his size. Trump is a bully in the most pathetic sense of the term.