If the U.S. invaded Greenland:
- Europe would not try to defeat America militarily
- Europe would make the invasion politically and economically impossible to sustain
- The real target would be the U.S. presidency itself, not the country
Above is aside from an invasion order never flying (requires Congress, violates War Powers Resolution, would be ‘killed’ by illegality triggering courts and military refusal, etc.), it would cause dramatic damage to the US (stockmarket, dollar status, borrowing costs). I think Europe can just laugh away Trumps threat but if you seriously believe he’ll follow through, then better to pull out investments now because they will take a serious hit.
Imagine Europe dumping treasuries, forcing sale of US assets, rigorously applying the law on the mag7, etc. - it would be an economic disaster.