Although the fighting took place simultaneously, the European theater and the Pacific theater of World War II had significant differences. In Europe, more nations were engaged with each other. On one side, Germany, Italy, and their puppet states faced off against the Soviets, Americans, Canadians, and British. This required more coordination between nations and the establishment of multi-national command structures. In some cases, such as the invasion of Normandy, the forces of several nations fought alongside each other. The Soviets on the eastern front were in charge of their own forces. However, after the Allies gained a foothold in Western Europe, it became necessary to coordinate the operations along that front through an organized command structure. Ultimately, General Eisenhower, an American, was designated as the "Supreme Allied Commander." The war in the Pacific, other than the fighting in mainland China, was mostly a conflict between Japan and the United States. Although Australia, New Zealand, and the British were involved to a certain extent, the United States conducted most of the fighting against the Japanese. Therefore, a complex multi-national command structure like the one in Europe was not necessary.
Geography also played a large role. The war in Europe was primarily fought on land. Both the Axis and Allies fought to capture and control land. The land itself was diverse, ranging from open deserts to alpine mountains, farmland, and forests. Fighting took place in cities and the countryside alike. This diversity in landscape put the focus mostly on infantry tactics. Air, armored, and naval forces supported the infantry, but it was soldiers with boots on the ground who did most of the fighting. The Pacific theater of the war was fought largely at sea and on small, far-flung islands. As a result, naval and air power became much more important. The use of battleships and aircraft carriers defined much of the combat in the Pacific. Soldiers fought on islands, where the landscape and tropical diseases often became as much of a threat as the enemy.
While the Germans were certainly capable of war crimes and atrocities (and did commit them frequently), they were more of a traditional enemy for the Allies. The Germans were also technologically proficient. The Allies in Europe had to face off against superior tanks, airplanes, and even ballistic missiles. The Japanese were not as technologically advanced as the Allies. Their technological deficits included a lack of radar and long-range bombers. However, they made up for this by being tenacious fighters, who usually fought to the death instead of surrendering.