Once Trump steals Greenland he is going to look at a map with the mercator projection of Greenland, Alaska and the continental united states and he will think ‘Canada is just right there in the middle; wouldn’t it be beautiful if they were all the united states.’ He will then commence serious moves to take Canada.
Europe and America’s own institutions have been feckless in preventing Trump’s blatant illegality. I think it better to ask what going to stop trump from taking Greenland.