The first draft imposed since the Civil War, the Selective Service Act of 1917, passed by overwhelming majorities in Congress, was signed by President Wilson a century ago. The Act provided for the enlistment, at the discretion of the President, of the four volunteer divisions that Theodore Roosevelt planned to lead. Go here to read about this provision. Wilson, alarmed that Roosevelt would either be killed in France and he would be blamed, or that he would come back a national hero and be swept into the Presidency in 1920, would refuse to ever authorize the four volunteer divisions. By the end of the War some 2 million Americans volunteered for service and some 2.8 million were drafted.
Individuals who belonged to religions or organizations opposed to War were exempted from combatant service but not from noncombatant service. Members of the clergy were exempted from conscription as were seminarians.