The mobilization of European armies in the opening months of World War I revealed the sheer size that the war would take. Armies numbering hundreds of thousands with the latest military technology were about to clash across Europe. As the United States witnessed these unfolding events, a small group of military officers, politicians, and other affluent individuals took to the public with a dire message; the US military was woefully ill prepared to defend itself, much less mount an expeditionary force to France. This push to improve the readiness of the US military including training and mandatory service was named the Preparedness Movement. Notables such as former Chief-of-Staff General Leonard Wood and former President Theodore Roosevelt argued that the US Army lacked the manpower, infrastructure, and sufficient training for defense. Former Secretaries of War Elihu Root and Henry Stimson publicly advocated for an increase in military spending and officer training schools. This enthusiasm for bolstering national defense was matched by a strong opposition. President Woodrow Wilson advocated for an armed neutrality rather than increase military spending.
The Preparedness Movement initially gained traction with prominent industrialists and politicians who believed in mediating international affairs via strong military. The internationalism focus ran counter to isolationist groups who not only wanted to remain neutral, but claimed that some Preparedness Movement proposals would resemble European armies they wanted to avoid, e.g. Germany. This situation gradually changed in 1915 and 1916 with two events; the Pancho Villa raid across the US-Mexico border and the sinking of the Lusitania. US national defense was tested, showing how the military faced numerous obstacles in ensuring their protection. Wilson and his Cabinet embarked on implementing a few of the programs supported by the Preparedness Movement, including a larger navy. By June 1916, Congress passed then National Defense Act which expanded the Army and National Guard and implemented the Reserve Officer Training Corps (ROTC). With passage of the NDA, the Preparedness Movement achieved much of its objectives and dissipated.
While controversial in its time, the Preparedness Movement pushed for the military readiness its members believed was crucial to ensure their protection and intervene abroad.
(Woodrow Wilson in a Preparedness Parade in Washington D.C., June 1916, https://catalog.archives.gov/id/23922795 )