On June 22, 1941, Germany, along with other European Axis members and Finland, invaded the Soviet Union in Operation Barbarossa. The primary targets of this surprise offensive were the Baltic region, Moscow and Ukraine, with an ultimate goal of ending the 1941 campaign near the A-A line, the line connecting the Caspian and White Seas. Hitler's objectives were to eliminate the Soviet Union as a military power, exterminate Communism, generate so-called 'living space' by dispossessing the native population and guarantee access to the strategic resources needed to defeat Germany's remaining rivals. Although before the war the Red Army was preparing for strategic counter-offensives, Barbarossa forced the Soviet supreme command to adopt a strategic defence. During the summer, the Axis made significant gains into Soviet territory, inflicting immense losses in personnel and matériel. However, by the middle of August, the German Army High Command decided to suspend the offensive of a considerably depleted Army Group Center, and to divert the Second Panzer Group to reinforce troops advancing toward central Ukraine and Leningrad. The Kiev offensive was overwhelmingly successful, resulting in encirclement and elimination of four Soviet armies, and made further advance into Crimea and industrially developed Eastern Ukraine (the First Battle of Kharkov) possible.
Khreshchatyk, the main street of Kiev, after German bombardment.
The diversion of three quarters of the Axis troops and the majority of their air forces from France and the central Mediterranean to the Eastern Front prompted the United Kingdom to reconsider its grand strategy. In July, the UK and the Soviet Union formed a military alliance against Germany and shortly after jointly invaded Iran to secure the Persian Corridor and Iran's oilfields. In August, the United Kingdom and the United States jointly issued the Atlantic Charter.
By October, when Axis operational objectives in Ukraine and the Baltic region were achieved, with only the sieges of Leningrad and Sevastopol continuing, a major offensive against Moscow had been renewed. After two months of fierce battles, the German army almost reached the outer suburbs of Moscow, where the exhausted troops were forced to suspend their offensive. Despite impressive territorial gains, the Axis campaign had failed to achieve its main objectives: two key cities remained in Soviet hands, the Soviet capability to resist was not broken, and the Soviet Union retained a considerable part of its military potential. The blitzkrieg phase of WWII in Europe had ended.
By early December, freshly mobilized reserves allowed the Soviets to achieve numerical parity with Axis troops. This, as well as intelligence data that established a minimal number of Soviet troops in the East sufficient to prevent any attack by the Japanese Kwantung Army,[ allowed the Soviets to begin a massive counter-offensive that started on December 5 along a 1000 km front and pushed German troops 100–250 km west.
Japanese troops advancing through Kuala Lumpur.
Japan had seized military control of southern Indochina the previous year, partly to increase pressure on China by blocking supply routes, but also to better position Japanese forces in the event of a war with the western powers. Japan, hoping to capitalize on Germany's success in Europe, made several demands, including a steady supply of oil, of the Dutch East Indies; these attempts, however, broke down in June 1941. The United States, United Kingdom and other western governments reacted to the seizure of Indochina with a freeze on Japanese assets, while the United States (which supplied 80% of Japan's oil) responded by placing a complete oil embargo. Thus Japan was essentially forced to choose between abandoning its ambitions in Asia and the prosecution of the war against China, or seizing the natural resources it needed by force; the Japanese military did not consider the former an option, and many officers considered the oil embargo an unspoken declaration of war. Japanese Imperial General Headquarters thus planned to rapidly seize European colonies in Asia to create a large defensive perimeter stretching into the Central Pacific; the Japanese would then be free to exploit the resources of Southeast Asia while exhausting the over-stretched Allies by fighting a defensive war. To prevent American intervention while securing the perimeter it was further planned to neutralize the United States Pacific Fleet from the outset.On December 7 (December 8 in Asian time zones), 1941, Japan attacked British, Dutch and American holdings with near simultaneous offensives against Southeast Asia and the Central Pacific. These included an attack on the American fleet at Pearl Harbor and landings in Thailand and Malaya.These attacks prompted the United States, United Kingdom, Australia, other western Allies and China (already fighting the Second Sino-Japanese War), to formally declare war on Japan. Germany and the other members of the Tripartite Pact responded by declaring war on the United States. In January, the United States, United Kingdom, Soviet Union, China and twenty-two smaller or exiled governments issued the Declaration by United Nations which affirmed the Atlantic Charter The Soviet Union did not adhere to the declaration, maintained a neutrality agreement with Japan and exempted itself from the principle of self-determination.Meanwhile, by the end of April 1942, Japan had almost fully conquered Burma, the Philippines, Malaya, the Dutch East Indies, Singapore, and the key base of Rabaul, inflicting severe losses on Allied troops and taking a large number of prisoners. Japanese forces also achieved naval victories in the South China Sea, Java Sea and Indian Ocean and bombed the Allied naval base at Darwin, Australia. The only real Allied success against Japan was a victory at Changsha in early January, 1942. These easy victories over unprepared opponents left Japan severely overconfident, as well as overextended.
Germany retained the initiative as well. Exploiting dubious American naval command decisions, the German navy ravaged Allied shipping off the American Atlantic coast.Despite considerable losses, European Axis members stopped a major Soviet offensive in Central and Southern Russia, keeping most territorial gains they achieved during the previous year. In North Africa, the Germans launched an offensive in January, pushing the British back to positions at the Gazala Line by early February] followed by a temporary lull in combat which Germany used to prepare for their upcoming offensives.