US President Donald Trump shows no signs of backing down on Greenland. In the latest, Trump has imposed 10% tariffs on countries that don't support American control of this semi-autonomous territory of Denmark, which is a NATO ally. The threats expose deep divisions within the alliance. The North Atlantic Treaty Organisation or NATO was formed in 1949 by 12 nations for collective security in Western Europe to deter Soviet expansion after the second world war. European leaders insist Greenland's future is for Denmark and Greenland to decide. The Greenland dispute is a test of NATO's unity like never before.