1939 to December 1941:
In the European Theatre, WWII began in September 1939 when Germany invaded Poland, quickly followed by Britain and France declaring war on Germany. Germany had already annexed Austria and the Sudetenland region of Czechoslovakia, but the invasion of Poland marked the beginning of a major war. In the following years, Germany invaded and conquered Denmark, Norway, Belgium, the Netherlands, and France. Britain was the only remaining major power in Europe that opposed Germany, and the two countries engaged in a protracted and bloody air campaign known as the Battle of Britain. The Soviet Union, led by Joseph Stalin, had initially signed a non-aggression pact with Germany, but this agreement ended in June 1941 when Germany invaded the Soviet Union, launching the largest military operation in history. The Soviet Union eventually became one of the major Allied powers in the war.
In the Pacific Theatre, the situation was less volatile in this time period. Japan was engaged in a war with China, which had been ongoing since the early 1930s, and had already occupied much of China. Japan also had territorial ambitions in Southeast Asia, but for the most part, it had not engaged in large-scale military operations in the region. The United States had imposed economic sanctions on Japan in response to its invasion of China, and tensions between the two countries were rising. However, the US was still officially neutral in the war in Europe and had not yet entered the conflict.
December 7, 1941 to June 1944:
In the European Theatre, the war was in full swing. Germany had occupied most of Europe and was now focusing on the Soviet Union. The tide began to turn against Germany in late 1942 when the Soviets began to push back, winning key victories at Stalingrad and Kursk. The Allies also launched a campaign in North Africa, where British and American forces defeated German and Italian forces in a series of battles. In 1943, the Allies invaded Italy and overthrew the fascist government of Benito Mussolini. Germany responded by occupying Italy and setting up a puppet government. Meanwhile, the Allies were planning a major invasion of France, which would take place in June 1944.
In the Pacific Theatre, the situation changed dramatically on December 7, 1941, when Japan launched a surprise attack on the US naval base at Pearl Harbor, Hawaii. The attack resulted in the US declaring war on Japan and entering WWII. Japan also began a campaign of territorial expansion, occupying much of Southeast Asia and the Pacific Islands. The US and its allies fought back, engaging in a series of naval and land battles, including the Battle of Midway and the Guadalcanal Campaign. The Allies gradually began to push back against Japan, but the war in the Pacific was far from over.