There are many differences between the European and Pacific theaters during World War II, including that the US entered the Pacific theater in response to the surprise attack at Pearl Harbor. According to the National World War II Museum,
On December 7, 1941, Japan staged a surprise attack on Pearl Harbor, severely damaging the US Pacific Fleet. When Germany and Italy declared war on the United States days later, America found itself in a global war. Japan launched a relentless assault that swept through the US territories of Guam, Wake Island, and the Philippines, as well as British-controlled Hong Kong, Malaya, and Burma. Yet, with much of the US fleet destroyed and a nation unprepared for war, America and its allies decided they needed to save Great Britain and defeat Germany first.
Conversely, the US deliberated for a long period of time over whether to join the Allies and enter the war in Europe. Before Germany and Italy declared war on the nation, President Roosevelt had many meetings with Winston Churchill, who was imploring him to bring US force to the European theater to help shorten the war.
Nevertheless, there are similarities between the two theaters. The war in both territories was difficult and deadly for the US. Unlike Germany, which had a large group of trained soldiers, the Japanese army had a small corps of officers who were professional military men. Most of the soldiers were relatively inexperienced. Even so, while the Japanese military might not have been as skilled or well-equipped as the German army, the Japanese fought aggressively.
Racial conflicts underpinned both fights. The war in Europe was launched by the German notion that German Aryans were superior to other peoples. The war in the Pacific was launched by the Japanese and also had racial overtones on both sides, although they might have been more nuanced.
Both theaters involved several countries. In the Pacific theater, the war was led by the Japanese against US and British territories. Moreover, the Japanese were fighting in partnership with the other Axis powers, Germany and Italy. In the European theater, the German-led fight encompassed military launches against Poland, France, Russia and many others. Similarly, the Germans were in agreement with the other Axis powers, Italy and Japan.
The war in both territories concluded when the Axis fighters were overpowered by the Allied forces. The war in the Pacific ended with the detonation of the atomic bomb, while the Nazis eventually were overcome by the approaching Russian and US military forces.