The Phoney War (French: Drôle de guerre; German: Sitzkrieg) was an eight-month period at the start of World War II, during which there was only one limited military land operation on the Western Front, when French troops invaded Germany's Saar district. Nazi Germany carried out the invasion of Poland on 1 September 1939. The Phoney period began with the declaration of war by the United Kingdom and France against Nazi Germany on 3 September 1939, after which little actual warfare occurred, and ended with the German invasion of France and the Low Countries on 10 May 1940. Although there was no large-scale military action by Britain and France, they did begin some economic warfare, especially with the naval blockade, and shut down German surface raiders. They created elaborate plans for numerous large-scale operations designed to cripple the German war effort. These included opening an Anglo-French front in the Balkans, invading Norway to seize control of Germany's main source of iron ore, and an embargo against the Soviet Union which supplied Germany's main source of oil. By April 1940, the lone execution of the Norway plan was considered inadequate to stop the German offensive.

The quiet of the Phoney War was punctuated by a few Allied actions. In the Saar Offensive in September, the French attacked Germany with the intention of assisting Poland, but it fizzled out within days and they withdrew. In November, the Soviets attacked Finland in the Winter War, resulting in much debate in France and Britain about an offensive to help Finland, but the forces finally assembled for this campaign were delayed until it ended in March. The Allied discussions about a Scandinavian campaign caused concern in Germany and resulted in the German invasion of Denmark and Norway in April, and the Allied troops previously assembled for Finland were redirected to Norway instead. Fighting there continued until June when the Allies evacuated, ceding Norway to Germany in response to the German invasion of France.

On the Axis side, the Germans launched attacks at sea in the autumn and winter against British aircraft carriers and destroyers, sinking several including the carrier HMS Courageous with the loss of 519 lives. Action in the air began on 16 October 1939 when the Luftwaffe launched air raids on British warships. There were various minor bombing raids and reconnaissance flights on both sides.