Denmark is refusing to allow the USA to purchase Greenland, with Danish EU representative Anders Vistisen telling Trump to "f**k off". Greenlanders don't want to be American either, so if Trump wants to annex Greenland it will have to be invasion.
Denmark is a foundation member of NATO, all members of which are sworn to defend each other against attack. This means that if the United States invades Greenland, the US military is under oath to fight against ... itself.
