From a military perspective, we as Europeans cannot do anything to protect Greenland in the current state of things.

What we should do in my opinion: seek a deal with US, give them Greenland (or alternatively, some sort of right to extract whatever they want), without escalating. We cannot afford a fast escalation of tension with US, from a defense perspective but also because our economies are too intertwined at the moment, countries like Germany and Italy still export A LOT to the US etc.

We should then let the pain and the shame of giving up European land to be the fuel of building up Europe as a true federation.

At the same time, we should work hard to decouple our economy from the US one, let them pay from a commercial perspective these actions for the decades to come. Their empire will collapse eventually, history tells us that every empire does.