On December 7, 1941, Japan attacked the United States at Pearl Harbor, bringing it too into the war on the Allied side. China also joined the Allies, as eventually did most of the rest of the world. China was in turmoil at the time, and attacked Japanese armies through guerilla-type warfare. By the beginning of 1942, the major combatants were aligned as follows: the British Commonwealth, the United States, and the Soviet Union were fighting Germany and Italy; and the British Commonwealth, China, and the United States were fighting Japan. The United Kingdom, the United States, the Soviet Union and China were referred as a "trusteeship of the powerful" during the World War II  and were recognized as the Allied "Big Four" in Declaration by United Nations These four countries were considered as the "Four Policemen" or "Four Sheriffs" of the Allies power and primary victors of World War II. From then through August 1945, battles raged across all of Europe, in the North Atlantic Ocean, across North Africa, throughout Southeast Asia, throughout China, across the Pacific Ocean and in the air over Japan.
Where did Japan attack the United States?
Pearl Harbor