During the five years of German 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 programme (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. Every December Norway gives a Christmas tree to the United Kingdom as thanks for the British assistance during the Second World War. A ceremony takes place to erect the tree in London's Trafalgar Square.