party Nasjonal Samling, Vidkun Quisling, tried to seize power but was forced by the German occupiers to step aside. Real power was wielded by the leader of the German occupation authority, Reichskommissar Josef Terboven. Quisling, as minister president, later formed a collaborationist government under German control. Up to 15,000 Norwegians volunteered to fight in German units, including the Waffen-SS.
There were also many Norwegians, and those of Norwegian descent, who joined the Allied forces as well as the Free Norwegian Forces. From the small group that had left Norway in June 1940 consisting of 13 ships, five aircraft and 500 men from the Royal Norwegian Navy who followed the King to the United Kingdom the force had grown by the end of the war to 58 ships and 7,500 men in service in the Norwegian Navy; 5 squadrons of aircraft (including Spitfires, Sunderland flying boats and Mosquitos) in the newly formed Norwegian Air Force; and land forces including the Norwegian Independent Company 1 and 5 Troop as well as No. 10 Commandos.
During the five years of Nazi occupation, Norwegians built a resistance movement which fought the German occupation forces with both civil disobedience and armed resistance including the destruction of Norsk Hydro's heavy water plant and stockpile of heavy water at Vemork, which crippled the German nuclear program (see: Norwegian heavy water sabotage). More important to the Allied war effort, however, was the role of the Norwegian Merchant Marine. At the time of the invasion, Norway had the fourth largest merchant marine fleet in the world. It was led by the Norwegian shipping company Nortraship under the Allies throughout the war and took part in every war operation from the evacuation of Dunkirk to the Normandy landings. Each December Norway gives a Christmas tree to the United Kingdom as thanks for the British assistance during World War II. A ceremony takes place to erect the tree in London's famous Trafalgar Square
