How and why did marijuana become illegal?
The United States began its war on marijuana in the early 20th century when the District of Columbia first placed restrictions on cannabis use in 1906. The United Kingdom criminalized it throughout most of its colonies by 1925; and internationally, most nations allowed importation or manufacture for "medical or scientific purposes" only. Some of America's wealthiest men were involved in the passage of the Marihuana Tax Act of 1937, which
... passed in order to destroy the hemp industry, largely as an effort of businessmen Andrew Mellon, Randolph Hearst, and the Du Pont family. With the invention of the decorticator, hemp became a very cheap substitute for the paper pulp that was used in the newspaper industry. Hearst felt that this was a threat to his extensive timber holdings. Mellon... the wealthiest man in America, had invested heavily in DuPont's new synthetic fiber, nylon, and considered its success to depend on its replacement of the traditional resource, hemp. (Wikipedia, Cannabis/History)
Since the mid-20th century, the U.S. (and other countries) have stressed the "Gateway Theory," the belief that marijuana use leads to the usage of stronger drugs. However,
Studies have shown that tobacco smoking is a better predictor of concurrent illicit hard drug use than smoking cannabis. (Wikipedia, Cannabis/Gateway drug theory)
Cannabis ranks as the least addictive of America's most popular recreational substances: Nicotine was most addictive, followed by cocaine, alcohol, heroin, caffeine and marijuana. Because of marijuana's purported effects--"psychoactive and physiological," "psychedelic," and "hallucinatory"--the drug has maintained an illegal status in most parts of the world until recently, when the states of Washington and Colorado passed laws legalizing its recreational use.
I began researching this question for a philosophy paper. I learned that the marijuana was legal and other than the widespread use of marijuana in the 1960's, it was used for medicinal purposes until Nixon signed the Controlled Substance Act in 1970 naming cannibis as a controlled substance. The Drug Enforcement Administration and the Food and Drug Administration determine what substances are added or removed from the various scheduled substances. My paper is actually more about the medicinal purposes and the question I had hoped to learn was whether or not there was a specific person named to have established the benefits which I never found. I guess it really does go back to something Indians introduced with the peace pipe and medicine men. I hope this answers your question.