Sweden's doctrine of neutrality is often traced back to the 19th century as the country has not been in a state of war since the end of the Swedish campaign against Norway in 1814. During World War II Sweden joined neither the allied nor axis powers. This has sometimes been disputed since in effect Sweden allowed in select cases the Nazi regime to use its railroad system to transport troops and goods, especially iron ore from mines in northern Sweden, which was vital to the German war machine. However, Sweden also indirectly contributed to the defence of Finland in the Winter War, and permitted the training of Norwegian and Danish troops in Sweden after 1943.