Canadian Prime Minister Mark Carney just confirmed that if the US was to invade Greenland, Canada would view it as an attack on Canada, would be obligated to assist in Greenland's defense, and could use military force against the United States.
“The future of Greenland is a decision for Greenland and for the kingdom of Denmark. We are NATO partners with Denmark and so our full partnership stands. Our obligations on Article 5, an Article 2 of NATO stand and we stand fully squared behind those.”