Complex sentence: This Administration had been preparing for war for a while by the time of Pearl Harbor, but once the war began, numerous changes occurred, primarily in the form of a plethora of new agencies to meet wartime needs.
Split into simpler sentences: This Administration had been preparing for war for a while by the time of Pearl Harbor. Once the war began, numerous changes occurred, primarily in the form of a plethora of new agencies to meet wartime needs.