Oh, I can answer this one!

The illegalization of cannabis began in 1906, with the Pure Food and Drug Act. The Act, among other things, heavily restricted the sale of patent medicines, medicines being sold over-the-counter, without real knowledge of their contents or effectiveness. The Act labeled many drugs as "poisons," mainly Opium, Alcohol (Prohibition Era, of course), Morphine, and of course, cannabis. They were officially known as "habit forming drugs." This was the beginning of its criminalization.

In California, in 1907, the Poison Act(1) was passed. It was amended 3 times after it's initial passing, and was used to regulate sale of hemp extracts. Cannabis was never initially targeted, however a revision in 1915 officially banned cannabis.

In 1925, a development in cannabis' eventual prohibition, Indian hemp was banned(2) for export in the United States.

Regulation continued, in Massachusettes in 1911, Maine and New York in 1914. In New York, cannabis was regulated using the Towns-Boyland(3) act, which restricted sale, and prohibited refills. Sale was also prohibited to those who already were known to have a habit.

In 1930, the Federal Narcotics Bureau began to pursue cannabis, due to the rising Prohibition movements of the era, mainly spearheaded by Harry J. Anslinger(4), with many propaganda films being produced by the bureau.

In 1937, Marijuana was banned for all purposes on a federal level, excluding medicinal and industrial (hemp), and was heavily taxed, in the Marijuana Tax Act.(5) Mandatory sentences we're required for possessing Marijuana (any amount).

Finally, in 1969, the Marijuana Tax Act was ruled unconstitutional, under the 5th Amendment, and was therefore repealed. Only a year after, in 1970, the Controlled Substances Act was passed(6), which officially, once and for all, completely prohibited use of cannabis in the United States, however, there was no longer a mandatory sentence for possession of small amounts of cannabis.

I hope that answered your question! Sources:

Gieringer, Dale H. (1999). "The Forgotten Origins of Cannabis Prohibition in California" (PDF). 1

"Topics of the Times". New York Times. New York City. July 30, 1914. 2

"W.W. WILLOUGHBY: OPIUM AS AN INTERNATIONAL PROBLEM, BALTIMORE, THE JOHNS HOPKINS PRESS, 1925". Druglibrary 3

McWilliams, John C. (1990). The Protectors: Harry J. Anslinger and the Federal Bureau of Narcotics, 1930–1962. Newark, Delaware: University of Delaware Press. p. 183. ISBN 978-0-87413-352-3. 4

"Full Text of the Marihuana Tax Act as passed in 1937". Schaffer Library of Drug Policy. Retrieved May 15, 2007. 5

Pub. L. No. 91-513, 84 Stat. 1236. October 27, 1973. 6