Large crowds gathered in Copenhagen to protest US President Donald Trump’s push to take over Greenland, an autonomous Danish territory. As both Denmark and the United States are members of the NATO alliance, any US aggression against another member state raises profound questions about the bloc’s future. If the United States were to invade Greenland, “this would essentially mean the end of NATO as we know it,” said Guntram Wolff, a senior fellow at Bruegel.