The National Defense Act is signed into law, increasing the size of the United States National Guard by 450,000 men.

The National Defense Act of 1916, Pub. L. 64–85, 39 Stat. 166, enacted June 3, 1916, was a United States federal law that updated the Militia Act of 1903, which related to the organization of the military, particularly the National Guard. The principal change of the act was to supersede provisions as to exemptions. 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 material.

The National Guard is a state-based military force that becomes part of the U.S. military's reserve components of the U.S. Army and the U.S. Air Force when activated for federal missions. It is a military reserve force composed of National Guard military members or units of each state, the territories of Guam, the U.S. Virgin Islands, Puerto Rico, and the District of Columbia, for a total of 54 separate organizations. It is officially created under Congress's Article I, Section 8 enumerated power to "raise and support Armies". All members of the National Guard are also members of the organized militia of the United States as defined by 10 U.S.C. § 246. National Guard units are under the dual control of U.S. state governments and the U.S. federal government.