We already did, 52 years ago:

Project 100,000, also known as McNamara's 100,000, McNamara's Folly, McNamara's Morons, and McNamara's Misfits, was a controversial 1960s program by the United States Department of Defense (DoD) to recruit soldiers who would previously have been below military mental or medical standards. Project 100,000 was initiated by Defense Secretary Robert McNamara in October 1966 to meet the escalating manpower requirements of the American government's involvement in the Vietnam War. According to Hamilton Gregory, author of the book McNamara's Folly: The Use of Low-IQ Troops in the Vietnam War, inductees of the project died at triple the rate of other Americans serving in Vietnam and following their service had lower incomes and higher rates of divorce than their non-veteran counterparts. The project was ended in December 1971 and has been the subject of controversy, especially during the manpower shortages of the Iraq War.