1. European victory was accomplished through the size of the Soviet force, American military might, Germany's flawed strategy of a two-front assault, and allied aerial bombardment. Although suffering brutal casualties throughout the war, the Soviet Union was able to withstand the advance of the German army, which wasn't anticipated by Hitler. Moreover, Hitler underestimated the might of the Soviets and was caught in a two-front conflict in the east with the Soviets and the west with the rest of the Allied forces. As the Allies advanced closer to Germany, aerial bombardments increased and crushed what little resources they had left.
2. In Europe, the invasion of Poland on September 1, 1939, is generally considered the start of the war. Britain and France immediately declared war. Germany had been expanding into territories they felt were inherently German. Although the United States remained "neutral" for the beginning of the war, it helped the Allied effort with the Lend-Lease agreement, which provided American resources to the Allies.
The Asian front began with the Japanese surprise attack on Pearl Harbor on December 7th, 1941. This event forced the United States to "officially" enter the war. Japan's aggression in Asia had been going on for a while at that point, with invasions in China and multiple brutal attacks on Chinese villages.
3. The war had a unifying effect on American society. Nearly all aspects of the American economy pitched in with the war effort. Businesses began to create materials for the war in areas they'd never worked with before. Women entered the workforce in unprecedented numbers as the men went overseas. This changed the dynamic of the American workforce totally. Rosie the Riveter became a staple image of American society in the attempt to recruit women to the workforce. It worked, and female workforce participation rose nearly ten percent.
4. The turning point in the Pacific theater was the Battle of Midway. Up until that point, Japan had control of the various islands in the Pacific. Japan also overestimated their destruction of the American naval fleet. Midway saw mass casualties and damage to the Japanese Navy, while the United States remained intact. The United States was then able to continue "island hopping" until they reached mainland Japan.
5. The Allied forces, led by Churchill, FDR, and Stalin, met to shape the post-war world in Europe and Asia. One specific event was the creation of the United Nations. This was in an attempt to create stable peace in the world by strengthening treaties and agreements with countries as well as ensuring aggressive behaviors like those of Germany and Japan would be stopped much earlier. Economically, the Marshall Plan, named for General George C. Marshall, was intended to help rebuild the devastated European economy. The United States gave around fifteen billion dollars in aid.