Step 1 : Introduction to the question "3. This event was probably known to be the one which led America to join the War. On which day was the bombing of Pearl Harbor?"
...1. 1 January 1940 2. 21 December 1941 3. 7 December 1941 4. 8 August 1945 This was date in which Japanese bombers attacked the American naval base in Hawaii. As a result, the Americans joined the war. Moreover, on 11 December 1941 Germany declared war on the USA.