With the Soviets wary of mounting tensions with Germany and the Japanese planning to take advantage of the European War by seizing resource-rich European possessions in Southeast Asia , the two powers signed the Soviet—Japanese Neutrality Pact in April Hitler believed that the United Kingdom's refusal to end the war was based on the hope that the United States and the Soviet Union would enter the war against Germany sooner or later. In November , negotiations took place to determine if the Soviet Union would join the Tripartite Pact.
The Soviets showed some interest, but asked for concessions from Finland, Bulgaria, Turkey, and Japan that Germany considered unacceptable. On 18 December , Hitler issued the directive to prepare for an invasion of the Soviet Union. They were joined shortly by Finland and Hungary. Hitler's objectives were to eliminate the Soviet Union as a military power, exterminate Communism, generate Lebensraum "living space"  by dispossessing the native population  and guarantee access to the strategic resources needed to defeat Germany's remaining rivals.
Although the Red Army was preparing for strategic counter-offensives before the war,  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 both personnel and materiel. By the middle of August, however, the German Army High Command decided to suspend the offensive of a considerably depleted Army Group Centre , and to divert the 2nd Panzer Group to reinforce troops advancing towards central Ukraine and Leningrad.
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.
By October Axis operational objectives in Ukraine and the Baltic region were achieved, with only the sieges of Leningrad  and Sevastopol continuing. The blitzkrieg phase of the war in Europe had ended. By early December, freshly mobilised reserves  allowed the Soviets to achieve numerical parity with Axis troops.
In , the United States had renounced its trade treaty with Japan, and beginning with an aviation gasoline ban in July , Japan became subject to increasing economic pressure. To increase pressure on China by blocking supply routes, and to better position Japanese forces in the event of a war with the Western powers, Japan invaded and occupied northern Indochina. Chinese nationalist forces launched a large-scale counter-offensive in early In August, Chinese communists launched an offensive in Central China ; in retaliation, Japan instituted harsh measures in occupied areas to reduce human and material resources for the communists.
German successes in Europe encouraged Japan to increase pressure on European governments in Southeast Asia. The Dutch government agreed to provide Japan some oil supplies from the Dutch East Indies , but negotiations for additional access to their resources ended in failure in June The United States, United Kingdom and other Western governments reacted to this move with a freeze on Japanese assets and a total oil embargo. Since early the United States and Japan had been engaged in negotiations in an attempt to improve their strained relations and end the war in China.
During these negotiations Japan advanced a number of proposals which were dismissed by the Americans as inadequate. Frustrated at the lack of progress and feeling the pinch of the American-British-Dutch sanctions, Japan prepared for war. On 20 November a new government under Hideki Tojo presented an interim proposal as its final offer.
It called for the end of American aid to China and for lifting the embargo on the supply of oil and other resources to Japan. In exchange Japan promised not to launch any attacks in Southeast Asia and to withdraw its forces from southern Indochina. Japan 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. These attacks led the United States , United Kingdom , China, Australia and several other states to formally declare war on Japan, whereas the Soviet Union, being heavily involved in large-scale hostilities with European Axis countries, maintained its neutrality agreement with Japan.
On 1 January , the Allied Big Four  —the Soviet Union, China, the United Kingdom and the United States—and 22 smaller or exiled governments issued the Declaration by United Nations , thereby affirming the Atlantic Charter ,  and agreeing to not to sign a separate peace with the Axis powers. During , Allied officials debated on the appropriate grand strategy to pursue. All agreed that defeating Germany was the primary objective.
World War II
The Americans favoured a straightforward, large-scale attack on Germany through France. The Soviets were also demanding a second front. The British, on the other hand, argued that military operations should target peripheral areas to wear out German strength, leading to increasing demoralisation, and bolster resistance forces. Germany itself would be subject to a heavy bombing campaign.
An offensive against Germany would then be launched primarily by Allied armour without using large-scale armies. At the Casablanca Conference in early , the Allies reiterated the statements issued in the Declaration by the United Nations, and demanded the unconditional surrender of their enemies. The British and Americans agreed to continue to press the initiative in the Mediterranean by invading Sicily to fully secure the Mediterranean supply routes.
By the end of April , Japan and its ally Thailand had almost fully conquered Burma , Malaya , the Dutch East Indies , Singapore , and Rabaul , inflicting severe losses on Allied troops and taking a large number of prisoners. In early May , Japan initiated operations to capture Port Moresby by amphibious assault and thus sever communications and supply lines between the United States and Australia. The planned invasion was thwarted when an Allied task force, centred on two American fleet carriers, fought Japanese naval forces to a draw in the Battle of the Coral Sea.
With its capacity for aggressive action greatly diminished as a result of the Midway battle, Japan chose to focus on a belated attempt to capture Port Moresby by an overland campaign in the Territory of Papua. Both plans started in July, but by mid-September, the Battle for Guadalcanal took priority for the Japanese, and troops in New Guinea were ordered to withdraw from the Port Moresby area to the northern part of the island , where they faced Australian and United States troops in the Battle of Buna-Gona. By the start of , the Japanese were defeated on the island and withdrew their troops.
The first, an offensive into the Arakan region in late , went disastrously, forcing a retreat back to India by May Despite considerable losses, in early Germany and its allies stopped a major Soviet offensive in central and southern Russia, keeping most territorial gains they had achieved during the previous year. The Germans split Army Group South into two groups: The Soviets decided to make their stand at Stalingrad on the Volga. By mid-November, the Germans had nearly taken Stalingrad in bitter street fighting. The Soviets began their second winter counter-offensive, starting with an encirclement of German forces at Stalingrad ,  and an assault on the Rzhev salient near Moscow , though the latter failed disastrously.