Trump will head to Davos, Switzerland, amid rising tensions with NATO allies over his Greenland acquisition plans and threatened tariffs on European nations.

The time for flattering Donald Trump is over and Europe should hit back hard economically if the U.S. imposes tariffs on NATO allies that sent troops to Greenland, ex-NATO boss and former Danish Prime Minister Anders Fogh Rasmussen said on Tuesday.

TIME spoke to legal experts about whether Trump could legally pull the U.S. out of NATO and the wider implications.

President Donald Trump announced that the NATO countries who have sent small contingents of troops to Greenland will be charged a 10% tariff on all goods.

Russia’s Foreign Minister said Trump’s actions upended the Western concept of the “rule-based global order” that Russia has long criticized.

Denmark, Norway, Sweden, France, Germany, the United Kingdom, the Netherlands and Finland are casting a united front after Trump threatens tariffs.

Sen. Mark Warner (D-VA) told Special Report on Monday that President Donald Trump's "obsession" with securing a Nobel Peace Prize is shaping his foreign policy decisions and putting NATO at risk. BRET BAIER,

Greenland and Denmark are due to meeting with top White House officials to discuss the Arctic island's future.