What's possible isn't an end of NATO, it's an EU-US war, with citizens of one in the other being interned, ships being prevented from leaving ports, complete embargoes à la WWII etc.
If Denmark is invaded, we're at war, and I don't see how it can take any other form than this in the initial phases.
I don't think it's a real barrier. The real barrier is the US troops in Germany, who could cause huge disorder. Many computers would of course have to be kept off to prevent updates from wiping everything though, so the first few days would be extremely interesting. Maybe immediate internet shutdowns to prevent malware updates while this goes on is the right solution, with people simultaneously sent out to help companies and individuals save their computers by installing some pre-checked OS.
Presumably we would grind up our Android and Apple phones on the first day-- i.e. actually take them out and physically crush destroy them as completely as possible, or at least remove the batteries and never use them again.
The US isn’t just going to straight up invade anyway. There’s lots of political this and that but it does want to keep the current world order to some degree.
If we're going to have a war it will probably last at least half a decade, probably a little bit more. What you say of the EU could also be said of the US at the time of the attack on Pearl Harbour in 1941.
If we weren't open to the possibility, we wouldn't be sending tripwire forces to Greenland, as we currently are.
[0]: https://en.wikipedia.org/wiki/2020–2021_China–India_skirmish...
Presumably we're doing that because we're willing to have a war.
My understanding is that most of the larger armies in the world, and nuclear weapons are there.
And significant portion of our trade and US bonds.