What was the conflict of World War 2?

I don't know how it started but I know USA enter because of Pearl Habor's incident.

This is from wikipedia click on 'related links' below to go to the page.

The war is generally considered to have begun on 1 September 1939, with the invasion of Poland by Nazi Germany and subsequent declarations of war on Germany by France and most of the countries of the British Empire and Commonwealth. Many countries were already at war by this date, such as Ethiopia and Italy in the Second Italo-Abyssinian War and China and Japan in the Second Sino-Japanese War.[3] Many that were not initially involved joined the war later in response to events such as the German invasion of the Soviet Union and the Japanese attacks on the U.S. Pacific Fleet at Pearl Harbor and on British overseas colonies, which triggered declarations of war on Japan by the United States, the British Commonwealth,[4] and the Netherlands.