NATO’s North Atlantic Treaty (1949) covers the territory of member states, and Denmark’s sovereignty over Greenland is recognised by the US itself in the 1951 Defence of Greenland Agreement. An armed attack on Greenland would qualify as an attack on Denmark, triggering Article 5 consultations and potentially collective defence.
In response, Danish Prime Minister Mette Frederiksen has warned that an armed US attack to take Greenland will end the 76-year-old Western military alliance. “Certainly, a military attack would be the end of NATO, because then Denmark would have to invoke Article 5 of NATO, which is the oath that countries have to come and protect us. Then the Americans and all other countries would be obliged to come and help Denmark protect Denmark against the United States. Of course, the Americans would veto that, and then NATO would be dead,” he said.