The war in Europe began in September 1939, when Germany, under Chancellor Adolf Hitler, invaded Poland. Britain and France responded by declaring war on Germany but took little action over the following months. In 1940, Germany launched its next initiative by attacking Denmark and Norway, followed shortly thereafter by attacks on Belgium, the Netherlands, and France. All of these nations were conquered rapidly.
Later in the summer of 1940, Germany launched a further attack on Britain, this time exclusively from the air. The Battle of Britain was Germany’s first military failure, as the German air force, the Luftwaffe, was never able to overcome Britain’s Royal Air Force.
As Hitler plotted his next steps, Italy, an ally of Germany, expanded the war even further by invading Greece and North Africa. The Greek campaign was a failure, and Germany was forced to come to Italy’s assistance in early 1941.
Later in 1941, Germany began its most ambitious action yet, by invading the Soviet Union. Although the Germans initially made swift progress and advanced deep into the Russian heartland, the invasion of the USSR would prove to be the downfall of Germany’s war effort. The country was just too big, and although Russia’s initial resistance was weak, the nation’s strength and determination, combined with its brutal winters, would eventually be more than the German army could overcome. In 1943, after the battles of Stalingrad and Kursk, Germany was forced into a full-scale retreat. During the course of 1944, the Germans were slowly but steadily forced completely out of Soviet territory, after which the Russians pursued them across eastern Europe and into Germany itself in 1945.
In June 1944, British and American forces launched the D-Day invasion, landing in German-occupied France via the coast of Normandy. Soon the German army was forced into retreat from that side as well. Thus, by early 1945, Allied forces were closing in on Germany from both east and west. The Soviets were the first to reach the German capital of Berlin, and Germany surrendered in May 1945, shortly after the suicide of Adolf Hitler.
The war in the Pacific began on December 7, 1941, when warplanes from Japan launched a surprise attack on the U.S. Navy base at Pearl Harbor, Hawaii. By this time, Japan had already been at war with China for several years and had seized the Chinese territory of Manchuria. After the Pearl Harbor attack, Japan began a massive campaign of expansion throughout the Southeast Asia–Pacific region.
Although the Pearl Harbor attack provoked a declaration of war by the United States on Japan the very next day, it would be several months before U.S. forces would get seriously involved militarily. In late spring of 1942, the United States and Japan engaged in a series of naval battles, climaxing in the Battle of Midway on June 3–6, 1942, in which Japan suffered a catastrophic defeat.
For the next year, the United States engaged Japan in a protracted struggle for the Solomon Islands, which lay near vital Allied shipping routes. Between August 1942 and February 1943, Allied forces carried out an invasion on the island of Guadalcanal—the beginning of a long series of Allied offensives that would eventually force the Japanese out of the Solomons and then pursue them from various other Pacific island chains that the Japanese had earlier seized. In the meantime, British and Indian forces were combating Japanese troops in Burma.
Fighting continued throughout the Pacific in 1944 and early 1945, including major battles at Leyte, Iwo Jima, and Okinawa. By the late spring of 1945, most of Japan’s conquests had been liberated, and Allied forces were closing in on the Japanese home islands. As they neared Japan proper, the Allies began heavy bombing campaigns against major Japanese cities, including Tokyo. This process continued through the summer of 1945 until finally, in early August, the United States dropped two atomic bombs on the cities of Hiroshima and Nagasaki. Stunned by the unexpected devastation, Japan surrendered a few days later.