Although some states drafted men during the War of 1812, James Madison's administration was unable to enact national conscription (which Daniel Webster, a Federalist opponent, denounced as an attempt at "Napoleonic despotism"), and it was not until the Civil War that the need to sustain massive armies brought a taste of national conscription to America. With a smaller population to draw upon, the Confederacy adopted the draft in 1862, eventually applying it to white males seventeen to fifty years of age. In all, 21 percent of the 1 million Confederate soldiers were conscripts. But by violating individual liberty and states' rights and by including unpopular class-bound occupational exemptions such as for overseers on large plantations, the Confederate conscription act engendered much discontent and considerable resistance.

[...]

Not until World War I did the United States rely primarily upon conscription. The Selective Service Act of 1917 was adopted in large part because a civilian-led "preparedness" movement had persuaded many Americans that a selective national draft was the most equitable and efficient way for an industrial society to raise a wartime army. Woodrow Wilson overcame considerable opposition, particularly from agrarian isolationists in the South and West and ethnic and ideological opponents of the war in the North, to obtain the temporary wartime draft.

[...]

The draft did not end with World War II. Except for a brief hiatus between 1947 and 1948, it helped maintain throughout the cold war a sizable number of men in the armed forces (a mix of volunteers, conscripts, and draft-induced volunteers). During the Korean War, 1.5 million men, eighteen to twenty-five, were drafted; another 1.3 million volunteered, primarily for the navy and air force. Discontent led to an increase in COs (the percentage of inductees exempted as COs grew to nearly 1.5 percent, compared to .15 in each world war). Some 80,000 draft evasion cases were investigated.

Conscription became one of the many casualties of the Vietnam War. After President Lyndon B. Johnson committed American ground troops in 1965, draft calls soared from 100,000 in 1964 to 400,000 in 1966, enabling U.S. forces there to climb from 23,000 military advisers in 1964 to 543,000 troops by 1968.

Although draftees were only a small minority (16 percent) in the American armed forces, they made up the bulk of the infantry riflemen in Vietnam (88 percent by 1969) and accounted for more than half the army's battle deaths. Because of student and other deferments, the draft and the casualties fell disproportionately upon working-class youths, black and white. African-Americans, 11 percent of the U.S. population, accounted for 16 percent of the army's casualties in Vietnam in 1967 (15 percent for the entire war).

when a draft is equitable, policy must take into account large segments of your voter base that may be pissed off by foreign policy.

Nixon reduced draft calls while gradually withdrawing U.S. troops, but his dispatch of American units across the border into Cambodia in 1970 led to massive public protests. Only reluctantly did Congress in 1971 extend the draft for two more years. The lawmakers also eliminated student deferments and voted a massive ($2.4 billion) pay increase for the lower ranks in order to achieve an avf by mid-1973. During the 1972 election campaign, Nixon cut draft calls to 50,000 and stopped forcing draftees to go to Vietnam. On January 27, 1973, the day a cease-fire was announced, the administration stopped drafting, six months before induction authority expired on July 1, 1973.