The conclusion best supported by the fact that the United States did not enter World War II until 1942 is:
U.S. leaders did not believe that the events of the war justified involvement until 1942.
This response reflects the prevailing sentiment at the time and the policies of the U.S. government, which were largely isolationist until the attack on Pearl Harbor in December 1941, leading to the U.S. formally entering the war in 1942.