Prior to U.S. involvement in World War II, Japan had been conquering land in Asia for almost a decade. The next target on their list was the Philippines, but the U.S. had troops stationed there, so Japan could not attack it without provoking a military response.
The Japanese decided to pre-emptively attack the U.S. naval base at Pearl Harbor in Hawaii, the likely origin of an American military response. After it was destroyed, the Japanese believed they could conquer the Philippines without any interference.
While the notion of an attack against the U.S. Naval fleet was unthinkable to most Americans, U.S. intelligence had information suggesting the Japanese might be up to something, but it was not known where or when. However, Frank Knox, U.S. Secretary of the Navy, made a statement on Dec. 4, 1941, to assure everyone that the situation was well in hand. “Whatever happens,” he said, “the U.S. Navy is not going to be caught napping.” The attack on Pearl Harbor occurred three days later.