The outbreak of World War II, and especially the German invasion and occupation of Norway, led Grieg to oppose Stalin's policies. In 1939, the Soviet Union signed the Molotov–Ribbentrop Pact with Nazi Germany and, until the 1941 Nazi invasion, instructed Communists worldwide to regard the ongoing war as an "Imperialist" war in which they should not take part. Grieg was a staunch anti-Nazi and Norwegian patriot and committed himself in 1940 to the struggle against the Nazi occupation. In the winter of 1939–40, Grieg served in the Norwegian Army in Finnmark on neutrality guard during the Russo-Finnish Winter War. In 1940, after serving during the Norwegian Campaign against German forces, he escaped to the United Kingdom aboard the same vessel that carried the Norwegian Royal family and the National Gold reserves.