The world wars were massive, international conflicts in the 1900s. World War One occurred from 1914-1918, and World War Two occurred from 1939-1945. The U.S. entered both wars late, because they feared a sneak attack. Both wars occurred in Europe, but World War II was also fought in the Pacific near Japan. Both wars were similar in nature, but they differed in weaponry, technology, and espionage.