Two years after former President Richard M. Nixon launched a war on drugs in 1971, calling substance use the nation’s “public enemy No. 1,” he made a startling admission during a meeting in the Oval Office. Speaking to a small group of aides and advisers at the White House in March 1973, Nixon said he knew that marijuana was “not particularly dangerous.” Nixon, who had publicly argued that curbing drug use globally warranted an “all-out offensive,” also privately expressed unease...