Okay I'm a teacher so let me explain in simple terms what happens the second the US invades Greenland:

1. NATO dissolves.
This would literally be a founding member of NATO invading another member nation. This would be a gross violation of one of our longest held treaties in our history. NATO as it exists, would not, and could not, survive.

2. Instant war.
This triggers the defense clause of the post-WW2 agreement, a clause specifically written to trigger if this exact thing were to ever happen. We are instantly at war with (at bare minimum!) the other 11 founding nations: Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, and the UK.

3. Financial ruin.
The EU is the largest holder of US foreign debt and it isn't even close. With every country now left defending itself, most of the EU decides that loaning money to a nation currently at war with them is a bad idea. They call in the debts, stop extending us credit, and within a week every US based business from Amazon to Walmart experiences an extinction level event.

4. We never, ever recover from this.
There is absolutely no way on God's green earth any nation ever trusts us ever again after this. How the fuck could they? Even if we get rid of Trump, even if we get rid of elected Republicans, even if we get rid of MAGA as an ideology itself, all we have proven in the end is that our friendship is capricious and can't be relied on because it could just as easily flip flop in another four years.

5. This isn't even including long term effects.
This is literally just what happens the very moment we send American troops to invade Denmark soil. We lose. That's what happens. We lose forever.