>Now imagine full-blown sanctions.
This would surely result in formal American withdrawal from NATO.
"The US invading Greenland would destroy NATO!", you say. I don't believe that the US would invade Greenland militarily; it will likely buy it, or obtain some sort of ironclad investment rights not dependent on whether Greenland is part of the Kingdom of Denmark or independent. But let's say that the US does, and NATO dissolves.
It comes down to net benefits. Would owning Greenland be more valuable for American national security, than the current NATO status quo of the US being willing to to accept its own cities being nuked if Russia invades Western Europe?
The calculus made more sense (if it ever did) during the Cold War, when NATO ended at Germany's eastern border. Does it make sense now, when Montenegro is a NATO member? I strongly suspect that the answer is not one that the rest of NATO would want to hear, regardless of the Ukraine War.