On September 3 at 11.15 GMT, the United Kingdom, Australia and New Zealand, followed six hours later by France, responded by declaring war on Germany, initiating a widespread naval war. South Africa (September 6) and Canada (September 10) followed suit. The Soviet Union joined the invasion of Poland on September 17.

Germany rapidly overwhelmed Poland, then Norway, the Netherlands, Belgium and France in 1940, and Yugoslavia and Greece in 1941. Italian, and later German, troops attacked British forces in North Africa. By summer of 1941, Germany had conquered France and most of Western Europe, but it failed to subdue the United Kingdom due to the resistance of the Royal Air Force and Royal Navy. Adolf Hitler then turned on the Soviet Union, launching a surprise attack (codenamed Operation Barbarossa) on June 22, 1941. Despite enormous gains, the invasion bogged down outside of Moscow in late 1941 as winter set in and made further advances difficult. The Germans launched another attack in the Soviet Union the following summer but the attack got bogged down in vicious urban fighting in Stalingrad. The Soviets later launched a massive counterattack encircling and then forcing the surrender of the German Sixth Army at the Battle of Stalingrad (1942–43), decisively defeated the Axis during the Battle of Kursk, and broke the Siege of Leningrad. The Red Army then pursued the retreating Wehrmacht all the way to Berlin, and won the street-by-street Battle of Berlin, as Hitler committed suicide in his underground bunker on April 30, 1945.

Meanwhile, the Western Allies successfully defended North Africa (1940–43), invaded Italy (1943), and then liberated France (1944), following amphibious landings in Normandy. Repulsing a German counterattack at the Battle of the Bulge in December, the Western Allies crossed the Rhine River and linked up with their Soviet counterparts at the Elbe River in central Germany. During the war in Europe, six million Jews, as well as Roma (Gypsies), Slavs, Communists, homosexuals, the disabled and several other groups, were murdered by Germany in a state-sponsored genocide that has come to be known as The Holocaust.

Japan launched virtually simultaneous surprise attacks against the major U. S. Navy base at Pearl Harbor, on Thailand and on the British territories of Malaya and Hong Kong. These attacks were on December 7, 1941 in western international time zones and on December 8 in the east. Later on December 8, Japan attacked The Philippines, which was controlled politically by the U.S. at the time and quickly fell to Japanese forces. On December 11, Germany and Italy also declared war on the United States. Japanese forces began their assaults on British and Dutch territory in Borneo on December 15. From their major pre-war base at Truk, in the South Pacific, Japanese forces began to attack and occupy neighboring Allied territories.

Japan had victory after victory in South East Asia and the Pacific, including the capture of 130,000 Allied prisoners in Malaya and at the fall of Singapore on February 15, 1942. Much of Burma, the Netherlands East Indies, the Australian Territory of New Guinea, and the British Solomon Islands also fell to Japanese forces.

The chief stated aim of the German policy at the time was the reacquisition of German territories taken by the Treaty of Versailles, and the addition of ethnic German regions of former Austria-Hungary to form a Greater Germany.

During one session with UK Prime Minister Neville Chamberlain, Hitler's aides brought him multiple reports alleging atrocities against ethnic Germans in nearby countries, which Hitler invoked in support of Germany's claims to its former territory.

Another of the main reasons that German society moved towards war was due to the perceived inequities of the Versailles Treaty. The Nazis claimed that only they could free Germany from international subjugation. Hitler remilitarized the Rhineland and the Ruhr, and overturned several territorial dispositions which were enacted by the treaty