World War II resulted in an estimated 55 million deaths worldwide. It was the largest and most destructive conflict in history. Germany started World War II by invading Poland on September 1, 1939. Britain and France responded by declaring war on Germany. German forces invaded western Europe in the spring of 1940. With German encouragement, the Soviet Union occupied the Baltic states in June 1940. Italy, a member of the Axis (countries allied with Germany), joined the war on June 10, 1940. From July 10 to October 31, 1940, the Nazis waged, and ultimately lost, an air war over England, known as the Battle of Britain.
After securing the Balkan region by invading Yugoslavia and Greece on April 6, 1941, the Germans and their allies invaded the Soviet Union on June 22, 1941, in direct violation of the German-Soviet Pact. In June and July 1941, the Germans also occupied the Baltic states. Soviet leader Joseph Stalin then became a major wartime Allied leader, in opposition to Nazi Germany and its Axis allies. During the summer and autumn of 1941, German troops advanced deep into the Soviet Union. On December 6, 1941, Soviet troops launched a significant counteroffensive. One day later, on December 7, 1941, Japan (one of the Axis powers) bombed Pearl Harbor, Hawaii, leading the United States to enter the war in alliance with Great Britain and the Soviet Union.
In May 1942, the British Royal Air Force carried out a raid on the German city of Cologne with a thousand bombers, for the first time bringing war home to Germany. For the next three years, Allied air forces systematically bombed industrial plants and cities all over the Reich, reducing much of urban Germany to rubble by 1945.
On the eastern front, during the summer of 1942, the Germans and their Axis allies renewed their offensive in the Soviet Union, aiming to capture Stalingrad on the Volga River, as well as the city of Baku and the Caucasian oil fields. The German offensive stalled on both fronts in the late summer of 1942. In November, Soviet troops launched a counteroffensive at Stalingrad and on February 2, 1943, the German Sixth Army surrendered to the Soviets. The Germans mounted one more offensive at Kursk in July 1943, the biggest tank battle in history, but Soviet troops blunted the attack and assumed a military predominance that they would not again relinquish during the course of the war.
In July 1943, the Allies landed in Sicily and in September went ashore on the Italian mainland. After the Italian Fascist Party's Grand Council deposed Italian premier Benito Mussolini (an ally of Hitler), the Italian military took over and negotiated a surrender to Anglo-American forces on September 8. German troops stationed in Italy seized control of the northern half of the peninsula, and continued to resist. Mussolini, who had been arrested by Italian military authorities, was rescued by German SS commandos in September and established (under German supervision) a neo-Fascist puppet regime in northern Italy. German troops continued to hold northern Italy until surrendering on May 2, 1945.
On June 6, 1944 (D-Day), as part of a massive military operation, over 150,000 Allied soldiers landed in France, which was liberated by the end of August. On September 11, 1944, the first U.S. troops crossed into Germany, one month after Soviet troops crossed the eastern border. In mid-December the Germans launched an unsuccessful counterattack in Belgium and northern France, known as the Battle of the Bulge. Allied air forces attacked Nazi industrial plants, such as the one at the Auschwitz camp (though the gas chambers were never targeted).
The Soviets began an offensive on January 12, 1945, liberating western Poland and forcing Hungary (an Axis ally) to surrender. In mid-February 1945, the Allies bombed the German city of Dresden, killing approximately 35,000 civilians. American troops crossed the Rhine River on March 7, 1945. A final Soviet offensive on April 16, 1945, enabled Soviet forces to encircle the German capital, Berlin. As Soviet troops fought their way towards the Reich Chancellery, Hitler committed suicide on April 30, 1945. On May 7, 1945, Germany surrendered unconditionally to the Western Allies at Reims and on May 9 to the Soviets in Berlin. In August, the war in the Pacific ended soon after the U.S. dropped atomic bombs on the Japanese cities of Hiroshima and Nagasaki, killing 120,000 civilians. Japan formally surrendered on September 2.