Obviously there is a gross disparity in military power between the US and Greenland (or Greenland + Denmark). It is unlikely that even all of (the rest of) NATO could win against the US, although of course if both sides committed heavily, both would lose a massive amount of personnel and materiel. The US Navy's lack of icebreakers (we have two, and they're ancient) could be a problem, given the location. But all-in-all the US would retain the force advantage.

Then the massive, absolutely brutal sanctions and ruthless economic warfare would kick in...and the US is oh so very screwed. Even if only NATO countries dump the US debt they hold and push the already-occurring movement to abandon the dollar as international reserve currency to completion, that will send the dollar into free-fall, resulting in Weimar Republic level inflation. Like, wipe out the purchasing power of your savings level stuff. And remember: any non-NATO country holding that debt will also have to dump it or face a huge loss. In some cases (Japan, for example) their economy isn't in a position to absorb a huge hit, and cutting losses by also dumping US debt would be basically forced on them, even if they're a staunch ally. The US economy is, in so many ways, a house of cards (so is China's, but that's another matter for another discussion).

NATO as currently constructed will cease to exist. The US will lose all military basing rights basically overnight. US "soft power" will evaporate. Those who advocate a new American isolationism will get their way...and quickly realize what an absolutely idiotic position they've taken (or in some cases not: they really do possess a combination of stubbornness and stupidity that potent.

This is an absolute crossroads for the US, and picking the wrong path would be legitimately catastrophic, to say nothing of profoundly unethical.