The law requires men to register with the government in order to be randomly for military service and was passed to congress in 1917

At the time of World War I, the US Army was small compared with the mobilized armies of the European powers. As late as 1914, the Regular Army had under 100,000 men, while the National Guard (the organized militias of the states) numbered around 115,000. The National Defense Act of 1916 authorized the growth of the Army to 165,000 and the National Guard to 450,000 by 1921, but by 1917 the Army had only expanded to around 121,000, with the National Guard numbering 181,000.