If they invade, I would guess they would (a) not bother to formally leave NATO, (b) just do it, and (c) have Ambassador Whitaker mount a Denial of Service attack on any Art 5 response.

Fortunately for anyone still believing in rule of law, there are not only non-NATO collective defence agreements already in place, but there are even non-collective-defence ways to trigger Art 21: https://legal.un.org/ilc/texts/instruments/english/draft_art...

Before the event? Nothing. Even without leaving NATO, they do not feel constrained by laws or treaties.

After invading? Absent a genuine casus belli (no BS like "lebensraum" or "I want it"), the consequences of invading will be being seen traitors, as dishonourable. This in turn means that everyone will immediately determine that no treaty they sign is worth the paper it's written on. It will also mean that nobody can trust that any money they lend to the US (either government or private) will be returned to them. This would force the government to pick something on a sliding scale between "print money to cover the 1.9 trillion USD deficit" to "delete social security entirely and make cutbacks to the military", this in turn triggers an immediate severe recession.

And that's just in isolation; given all the other policies Trump et al are pursuing at the same time, the probability of actual hyperinflation (≥50%/month) goes from "lol no" to "plausible".

I think enough people in positions of power in the US know this, to put moderate-to-high odds on the US invoking the 25th if those around him think he's serious, and without the 25th medium odds of there being a military coup to prevent an invasion.

But, as per questions about if the Russian nuclear deterrent is actually a threat or instead currently made of rust, the odds aren't favourable to want to risk it.