Harry Anslinger was a career bureaucrat working in alcohol prohibition at the Treasury Department. His discover of the evils of marijuana came about, coincidentally, right after the repeal of prohibition. Originally it was supposed to be a tax act, and hemp growers would purchase a federal tax stamp. That's what was presented to Congress, and what they voted to approve. It became a defacto prohibition when Anslinger refused to issue the stamps. Suddenly an army of embarassingly idle T-men found themselves back in the prohibition enforcement business.
The reference to FDR and the CC is because the Marihuana Tax Act, as well as the modern day Controlled Substances Act and the DEA itself rely wholly on FDR's New Deal interpretation of the Commerce Clause for their authority.
"There are 100,000 total marijuana smokers in the US, and most are Negroes, Hispanics, Filipinos and entertainers. Their Satanic music, jazz and swing, result from marijuana usage. This marijuana causes white women to seek sexual relations with Negroes, entertainers and any others."
Harry J. Anslinger, testimony to Congress, 1937