What is National Defense Act Of 1916?

Legal Definition
The National Defense Act of 1916, Pub.L. 64–85, 39 Stat. 166, enacted June 3, 1916, was a federal law that updated the Militia Act of 1903, which related to the organization of the military, particularly the National Guard. The 1916 act included an expansion of the Army and the National Guard, the creation of an Officers' and an Enlisted Reserve Corps, and the creation of a Reserve Officers' Training Corps. The President was also given expanded authority to federalize the National Guard, with changes to the duration and the circumstances under which he could call it up. The Army began the creation of an Aviation arm, and the federal government took steps to ensure the immediate availability of wartime weapons and equipment by contracting in advance for production of gunpowder and other materiel.
-- Wikipedia