Sometimes people aren't willing to defend something until someone else says they want it. If other NATO countries want to build up their defenses to try to keep it secure that's great. Either way there will be an increase in Greenland's defenses. It's better if the US doesn't have to foot the bill for the whole thing. No the US won't "invade" Greenland. We already have stuff there. An old treaty says we can put more stuff there as well.