In early 1940, the Allies approved a naval campaign designed to seize the northern part of Norway, a neutral country, including the key port of Narvik, and possibly also to seize the iron mines at Gällivare in northern Sweden, from which Germany obtained much of its iron ore. The British planned to invade Norway in March 1940 to use the ports as bases for the Royal Navy in order to encircle Germany, but the operation (codenamed Plan R 4) was postponed. The British and French had previously planned to invade Sweden, in order to assist Finland during the Winter War against the Soviet Union. Adolf Hitler found out about Plan R 4, and militarily intervened as the British began Operation Wilfred by laying mines in neutral Norwegian waters.
The plan to blow up the factory partially succeeded, but the factory managed to continue producing, until the USAAF finally bombed it in 1943.
During filming, Kirk Douglas and Richard Harris failed to get along with each other. The tension between the two was fairly noticeable to cast and crew alike.
The real-life World War II missions, on which this movie was based, were conducted by Norwegian members of the British Special Operations Executive (SOE), and resulted in the deaths of thirty British commando soldiers. Some were captured, interrogated, tortured, and shot by the Nazi Gestapo, while some of them died when two gliders crashed upon landing in Norway.
In the skiing chase, where the black-clad Jensen (Roy Dotrice) is pursuing Pedersen (Kirk Douglas), Dotrice's stand-in was one of the original saboteur heroes of the World War II Telemark operation.