The focus on the European theatre represented one of the major elements of the Allied plan to win the war. It consisted of full commitment to the European theatre in air and ground campaigns throughout Europe to push back Hitler. It was seen that in beating Hitler and setting down this threat, the Japanese could be defeated. The full commitment of the Allied powers along the lines of economics in terms of gearing all production towards war efforts and through social embrace of the war goals helped to ensure victory. The passage of draft laws in the United States also helped to deliver a steady supply of soldiers in order to deliver victory.
Before the Japanese bombed Pearl Harbor, many Americans were undecided (and against) our nation getting involved in another war in Europe. However, the surprise attack in Hawaii solidified the nation's resolve to join England in its fight against Germany and Japan and their allies. The British were well aware that once America brought its massive military and financial reserves into action that it was only a matter of time before Germany and Japan succumbed to overwhelming numbers. Germany's attack on their former allies, the Russians, brought Russia into alliance with the U.S. and Great Britain, providing another nation capable of supplying huge armies to attack Germany from the north. Once the D-Day Invasion succeeded, the plan was to defeat the Germans and Japanese by sea and air as well as on land. American naval superiority soon asserted itself in both the Atlantic and Pacific, and the once-mighty German Luftwaffe was also weakened significantly. Allied forces soon surrounded German armies from three sides, and Japan's military force eventually withdrew back to its home islands to brace for the expected Allied invasion.
The Allied Plan (or at least the plan that the British and Americans agreed to) for winning the war was called the Atlantic Charter. This was their statement of overall strategy.
The statement said that they would concern themselves first with Germany. They would do their best to defeat Germany by eventually invading Europe. In the meantime, they would just try to prevent Japan from gaining more ground in the Pacific. If they could push Japan back, okay, but if not, they would just hold the line until Germany could be defeated.
In a major and long drawn war It is neither possible or desirable to have a single plan to conduct the entire war. The basic warring sides in the war also kept during the course of war. USA did not participate in the war directly, and Russia was initially pursuing its own expansionist goals rather than oppose those of Germany. Than take the example of Russia not declaring war against Japan till August 8, 1945, when the war had almost ended. In addition to these political uncertainties there were the uncertainties of the battle ground itself. In situation like this it is not possible to identify a common policy pursued by allies during the entire or even most of World War II.