With a lot of discussion and debate.

"Germany First" was the primary policy and it was settled on as part of the "Victory Plan" that was decided upon before the war. The Victory Plan correctly determined that in the event of war Germany would be the greatest enemy among the Axis powers; as Germany had the greatest industrial resources and the potential to secure even more resources if it was able to defeat the Soviets. Hence as early as mid-1941 the US Army was already preparing plans for a cross-channel invasion to bring the fight to the Germans; and was already drafting hundreds of thousands of men to actually conduct the assault. America quite simply was not as "innocent" as people make it out to be and didn't have to be "awakened" into the conflict by the Pearl Harbor attack.

That said, while the official policy of the United States was "Germany First" the United States Navy - led primarily by Admiral King - argued persuasively that resources also had to be devoted to the Pacific to prevent the Japanese from establishing unassailable defenses in 1942. This resulted in the Pacific getting the lion's share of naval assets in 1942.

The Pacific also received a large boost in 1943 due to the postponement of the invasion of France. While Marshall and other US Army officials had wanted to invade France in 1943, the British were reluctant to commit to such a venture at this point. The compromise, made after the Casablanca Conference, settled on a more limited invasion of Italy instead. Indeed, part of the reason why Eisenhower had landing craft shortages in 1944 was because a lot of the landing craft originally earmarked for Europe in 1943 went to the Pacific to join the growing US Fleet.

Hence, by 1944, although the Normandy invasion now consumed most the army's resources, the US Pacific Fleet had grown to an extent that it was now able to strike deep into Japan's initial conquest and was invading some of the home islands by 1945.

The US Army Air Force meanwhile had its own plans, and focused on the strategic bombing campaign with the British over Europe from 1943 onwards. The problem here is that they had grossly overestimated the effect of bombing, which is why it largely failed to end the war on its own. It really wasn't until '44/'45 that they really began to coordinate and fix the earlier problems; and it's the disappointments of the bombing campaign that must rank as one of the bigger missed opportunities of the war.