To be more precise, Greenland is for staging an invasion of Svalbard and Jan Mayen, which are themselves for staging an invasion of mainland Norway. Some would argue that Trump is incapable of this sort of long-term thinking; and some would also argue that it's wholly unnecessary to have SJM for staging an invasion anyways, given that mainland Norway is already militarily dependent on Seppolandic technology and hosts a large number of Seppo soldiers on its territory capable of launching an invasion "from within" at a moment's notice. Such people, though their attempt at analysis is commendable, do not understand the core of international geopolitics.
(This comment is not a serious analysis)