US invading Greenland means all US troops in Europe risk getting captured/foxholed in their own military bases. That's 65k permanent staff and 20-35k on rotation. Or 10% of the US military, simply turning PoW (and a huge bargaining chip so early in the conflict).

It will also signal to the world that America is no longer world police, which means the Middle East, Japan and Taiwan will once again come under threat. The Middle East already foresaw this, which is why Saudi Arabia, Turkey and Pakistan are building a regional nuclear-enabled alliance, with open invitation to the other oil economies. Taiwan is most likely to get caught with their pants down, given the lack of US military support.

Interestingly, with the exist of the US from NATO, the largest military in NATO will be Turkey's, that country that Europe has long considered the unwanted bastard child.