The Atlantic is, for all intents and purposes, our ocean in terms of blue water naval strength. The European nations don't have nearly the same ability to project force and any military forces they sent across the Atlantic to attempt to reinforce or liberate Greenland would take a pounding from the Navy's Atlantic fleet. The USN would quickly establish air superiority, as well, and then there are who knows how many SSN attack subs lurking out there. God forbid Trump get any bright ideas with the boomer subs.
The only nation that would be in a position to do anything about it would be Canada, and the Canadian armed forces are by no means a match for the US. Under Article 5, though, they'd theoretically be obligated to do something about it.
If Trump goes through with it there's not a lot Europe could do about it, militarily. NATO would cease to exist in any practical sense, with either the other members leaving and forming their own treaty organization or attempting to boot the US out, assuming the charter allows for that. It would drive a stake right into the heart of the post-WWII world order.
If Trump is stupid enough to invade Greenland, it will irrevocably change the geopolitical world. Most unfortunately, even the tenuous guardrails that were in place during his first term are absent now. There's no Mattis or Tillerson to try to curb his stupidest impulses, only a yapping pack of sycophants and parasites. There's no chance of the Cabinet invoking the 25th Amendment and removing him from office, either.
The whole Trump/Greenland thing is essentially analogous to a small child gazing at a land mine while holding a hammer.
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