Military strategy
Through this period, and until the German invasion of the USSR in June 1941, there was no possibility of Britain winning the war alone, and so British Grand Strategy aimed to bring the USA into the war on the allied side. Prime Minister Churchill devoted much of his diplomatic efforts to this goal. In August 1941, at the Atlantic Conference he met US President Roosevelt in the first of many wartime meetings wherein allied war strategy was jointly decided.
In December 1941, following the Japanese attack on Pearl Harbor, the United States entered the war. Britain was now also at war with imperial Japan, whose forces inflicted rapid defeats on British forces in Asia, capturing Hong Kong, Malaya, Singapore and Burma. Nevertheless, Churchill expressed the view that with the entry of the USA into the war, ultimate victory was assured for the Allies. "All the rest was merely the proper application of overwhelming force". From this point onward, the strategy of the Allies, other than the USSR, is better addressed as joint Allied Strategy

European Allies: In the December 1941, at the Arcadia Conference, the Allied leaders agreed to the "Germany first" principle whereby Germany was to be defeated first, and then Japan. However, Allied land forces would not be capable of invading the mainland of Europe for years, even as Joseph Stalin pressed for the western allies to alleviate pressure on the Eastern front. Supporting the Soviet war effort was a significant element of Allied strategy, and significant aid was shipped to the USSR through the Lend-Lease programme.
Strategic warfare, and especially strategic bombing, was a supporting component of Allied strategy. Through 1942 and 1943, the Allies gradually won the war at sea and in the air, blockading Germany and subjecting it to a strategic bombing campaign of increasing effectiveness Strategic bombing during World War II.
In January 1943, at the Casablanca Conference, the Allies agreed to demand Axis unconditional surrender, a war aim which implied the physical occupation of Germany with land forces. While building up strength for an invasion of continental Europe, the Allies pursued an indirect strategy by invading Europe from the South. After defeating Axis forces in North Africa (the invasion of French North-Africa), Sicily and southern Italy were invaded, leading to the defeat of Fascist Italy. Churchill especially favoured a Southern strategy, aiming to attack the "soft underbelly" of Axis Europe through Italy, Greece and the Balkans in a strategy similar to the First World War idea of "knocking out the supports". Roosevelt favoured a more direct approach through northern Europe, and with the Invasion of Normandy in June 1944, the weight of Allied effort shifted to the direct conquest of Germany.
From 1944, as German defeat became more and more inevitable, the shape of post-war Europe assumed greater importance in Allied strategy. At the Second Quebec Conference in September 1944, the Allies agreed to partition and de-industrialize a defeated Germany so as to render it permanently unable to wage war (see Morgenthau Plan). After the war, this plan was abandoned as unworkable. At the Tehran Conference Allied strategy adopted its final major component with the acceptance of Soviet conditions for a sphere of influence in Eastern Europe, to include eastern Germany and Berlin.

Soviet: Early Soviet strategy aimed to avoid or delay war, while developing the central government's hold over the state and expanding the industrial base. Soviet economy and military was weak, but rapidly expanding in an intense industrialization process. The USSR had been overtly hostile to Nazi Germany for most of the pre-war period, but the failure of appeasement convinced Stalin that the Allies were actively seeking a Nazi–Soviet war. The Soviet government doubted that a war against Germany could be avoided. However, negotiations were continued in order to, at the very least, buy time and permit the Soviets to secure the Soviet–German border through expansion and pressure on strategically important states perceived as possible German allies in a future war. The signing of the Molotov–Ribbentrop pact gave the USSR freedom to, in its view, preempt hostile action from nations along its Western border.
The invasion in the Barbarossa campaign of 1941 came earlier than expected to the Soviet leadership, resulting in the catastrophic loss of over 4 million Soviet soldiers killed or captured. Nevertheless, the USSR managed to halt the German advance at the outskirts of Moscow and Leningrad. With spies providing the certain knowledge that Japanese forces in the far east would not attack Siberia, the Soviets were able to transfer large numbers of experienced forces from the far east, and in the Winter of 1941/1942 they used them to counter-attack the German Army Group Centre in front of Moscow.
As the army was being defeated and giving up ground in the initial assault, a gigantic operation was staged to move economic capacity from the Western areas that were about to be overrun, to Eastern regions in the Urals and central Asia that were out of reach of the Germans. Entire factories, including their labour force, were simply moved, and what couldn't be taken was destroyed ("scorched earth"). As a result, even though huge territories were captured by the Germans, the production potential of the Soviet economy was not correspondingly harmed, and the factories shifted to mass production of military equipment quickly. Even before the war, Soviet industrialization had brought Soviet GDP to a level roughly equivalent to Germany. Although a significant part of the urban population had been captured by Germany in the 1941 campaign, the Soviet economy immediately went to a total war footing and was soon outproducing the German economy in war materiel.
It quickly became apparent that the war in the east would be pitiless and total. Soviet strategy was therefore aimed at preserving the state, at whatever cost, and then the ultimate defeat and conquest of Germany. This strategy was successful. By 1943, the USSR was confident in final victory and new aim of Soviet strategy became securing a favourable post-war Europe. At the Tehran Conference of 1943, Stalin secured acquiescence to a Soviet sphere in influence from his western allies.

Japanese: Japanese World War II strategy was driven by two factors: the desire to expand their territories on the mainland of Asia (China and Manchuria), and the need to secure the supply of raw resources they didn't have themselves, particularly oil. Since their quest after the former (conquest of Chinese provinces) endangered the latter (an oil boycott by the US and its allies), the Japanese government saw no other option than to conquer the oil sources in South-East Asia. Since these were controlled by American allies, war with the USA was seen as inevitable; thus, Japanese leaders decided it would be best to deal a severe blow to the U.S. first. This was executed in the Pearl Harbor strike, crippling the American battle fleet.
Japan hoped it would take America so long to rebuild, by the time she was able to return in force in the Pacific, she would consider the new balance of power a fait accompli, and negotiate a peace. However, the attack on Pearl Harbor failed to destroy the crucial targets (aircraft carriers and, most crucially for Japan's ability to hold island bases, submarines) and ignored others (oil tank farms, power station), thus the U.S. Navy was not weakened enough to force withdrawal. The psychological effect also caused the U.S. population and armed forces to fully mobilize for war. South-East Asia was quickly conquered (Philippines, Indochina, Malaysia and the Dutch East Indies). After Japan's vital aircraft carrier force was destroyed in the Battle of Midway, the Japanese had to revert to a stiff defense they kept up for the remainder of the war.

American: With both Japan and the US fighting two-front wars (against each other in the Pacific, and additionally the US in Europe and the Japanese in China), the far greater American economic power enabled the US forces to replace battle losses considerably faster and to eventually outgun the Japanese. In several aircraft carrier battles, the initiative was taken from the Japanese, and after the Battle of Midway, the Japanese navy was rendered helpless, effectively giving the Americans vast naval superiority.
After the Japanese were forced into the defensive in the second half of 1942, the Americans were confronted with heavily fortified garrisons on small islands. They decided on a strategy of "island hopping", leaving the strongest garrisons alone, just cutting off their supply via naval blockades and bombardment, and securing bases of operation on the lightly defended islands instead. The most notable of these island battles was the Battle of Iwo Jima, where the American victory paved the way for the aerial bombing of the Japanese mainland, which culminated in the atomic bombings of Hiroshima and Nagasaki and the Bombing of Tokyo that forced Japan to surrender.

Australian: Australia's historical ties with Britain meant that with the commencement of World War II its armies were sent overseas to contribute to battles in Europe. Fear from the north was so understated that at the outbreak of open warfare with Japan, Australia itself was extremely vulnerable to invasion (possible invasion plans were considered by the Japanese high command, though there was strong opposition). Australia's policy became based entirely on domestic defense following the attacks on Pearl Harbor and British assets in the South Pacific. Defying strong British opposition, Australian Prime Minister John Curtin recalled most troops from the European conflict for the defense of the nation.
Australia's defensive doctrine saw a fierce campaign fought along the Kokoda track in New Guinea. This campaign sought to further stretch Japanese supply lines, preventing the invasion of the Australian mainland until the arrival of fresh American troops and the return of seasoned Australian soldiers from Europe.