Congress passed the acts to prepare Americans for the possibility of U.S. involvement in the war by expanding the size of army and navy construction.
By the time these federal laws were enacted, the WW II was taking place in Europe, and the U.S. government wanted to be prepared in case they decided to enter the war. The National Defense Act allowed for the increase of the size of the Army and the National Guard as well as for the creation of an Officers' and an Enlisted Reserve Corps, and a Reserve Officers' Training Corps. The Naval Construction Act allowed for the enlargement of the U.S. Navy in a three-year period.