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1940–1941 refers to a two-year span within World War II that saw rapid expansion of Axis military power in Europe and Asia, along with the widening of the war into the Pacific. The period begins with swift German victories in Western Europe and ends with major turning points in 1941, including the entry of the United States into the conflict and the opening of the Eastern Front.

In 1940, Germany executed a rapid campaign in the Nordic countries and Western Europe, invading Denmark and Norway in April, followed by the invasion of the Low Countries, Luxembourg, and France in May–June. France capitulated in June, and an armistice led to the division of the country and the establishment of the Vichy regime. The Battle of Britain, fought in the summer and autumn of 1940, saw the United Kingdom resist a sustained aerial assault by the German Luftwaffe. Italy joined the war, extending Axis operations to the Mediterranean and North Africa. The period also followed the conclusion of the Winter War between the Soviet Union and Finland in early 1940, after which the Soviet Union prepared for future conflicts rather than Western European campaigns.

In 1941, the conflict widened further. Germany and its allies launched Operation Barbarossa, a massive invasion of the Soviet Union beginning on June 22, 1941, opening the Eastern Front. The Balkans campaign brought Axis occupation to Greece and Yugoslavia in the spring. In the Pacific, Japan accelerated expansion, culminating in the attack on Pearl Harbor on December 7, 1941, which led the United States to enter the war. The year also saw Britain and the United States adopt measures such as the Lend-Lease Act and the Atlantic Charter, while the siege of Leningrad began and North African campaigns intensified.