Donald Trump has presented NATO with an existential dilemma. The U.S. president has long been attacking the military organization. He has even gone so far as to question its very core, its fundamental principle: the mutual defense clause. Now, the White House's threats against Greenland, an autonomous territory that is part of Denmark a NATO member like the United States without ruling out military action, could shatter the transatlantic alliance on which Europe has relied for its security since the end of World War II.