Nixon famously inaugurated the so-called War on Drugs in 1970 with the Controlled Substances Act (CSA), which regulated certain drugs, divided into tiers based on the level of danger they posed. In 1973, Nixon founded the Drug Enforcement Administration (DEA), whose job it is to control the the smuggling of illegal drugs into the US. The ostensible reason for this was that Nixon considered drugs a "public enemy," since recreational drug use had risen in the decade before Nixon assumed office in 1969.
In the past decade, a quote from Nixon's domestic policy chief, John Ehrlichman, claimed that Nixon's War on Drugs was in fact motivated by a covert effort to target black and anti-war activities (the counterculture group popularly known as "hippies"). According to Ehrlichman (who was interviewed by journalist Dan Baum for Harper magazine), blacks were associated with heroin use and hippies with marijuana. The merit of this quote has been questioned by Ehrlichman's family, though critics of Nixon's policies are eager to cite it.
The cultural backdrop against which this took place was the Vietnam War, a two-decade-long campaign that the US joined in 1965. US involvement in Vietnam was hugely controversial, leading to the the hippie movement. The war (which the US eventually lost) resulted in nearly 60,000 US casualties, and hippies showed their dissent by promoting peace. Their image was one of free love and drugs—especially marijuana.
Nixon may have used the War on Drugs to help his campaign by means of destabilizing groups of blacks and hippies who did not directly endorse him. Those skeptical of Nixon's policies point to the disproportionate number of blacks imprisoned as a result of the drug use. Indeed, his drug laws increased the incarceration rates dramatically, and to this day, about sixty percent of those in federal prisons are either black or Latino.