"We aim above the mark to hit the mark."
Ralph Waldo Emerson
"Whenever people are well-informed they can be trusted with their own government."
"Prohibition goes beyond the bounds of reason in that it attempts to control a man's appetite by legislation, and makes a crime out of things that are not crimes."
A Brief History of Cannabis Prohibition - Part 3By Drew Whitworth
In 1912, an international conference at Den Haag (the Hague) was the first to seriously discuss the "need" for the confining "to medical and legitimate purposes the manufacture, sale and use of opium, heroin, morphine and cocaine". Cannabis was not included at this stage, though the conference considered it - and though a handful of US states had banned its use prior to the 1920s, at the time of alcohol prohibition across the US, most states still permitted the use of cannabis.
So why the subsequent rapid sea-change? As I've said above, the motives were far more economic and political than any concern for the "welfare" of smokers. South Africa, a prohibition pioneer, stated at the League of Nations in 1923 that cannabis should be prohibited because it affected the productivity of (black) mine-workers. Cannabis was declared a "narcotic" the next year. Also around this time, improved technology and the rise of large petrochemical corporations - DuPont being the most notable example here - encouraged these large corporate interests to start developing and marketing alternatives to hemp, such as plastics and paper prepared from wood-pulp. Though there were over 1,000 hash bars in New York City at this time, the drug was still associated mainly with black and Hispanic immigrants, and therefore seen as implicitly undesirable.
Britain made cannabis illegal in September 1928, and in 1931, the US formed the Federal Bureau of Narcotics. Its head, Anslinger, immediately launched a vitriolic offensive on cannabis, calling it at one point "the most violence-causing drug known to man". Over the objections of the American Medical Association, in 1937 the US passed the infamous Marijuana Tax Act which forbade hemp farming. The Act was based on the Machine Gun Transfer Act which made it illegal to pass on machine guns without a government stamp - there being no such stamps available. By applying this strategy to marijuana, Anslinger was able to effectively ban hemp without contravening constitutional rights.
"Coincidentally", immediately after the passing of this act, DuPont filed patents for nylon, plastics and a new bleaching process for paper, thus taking advantage of the gap in the market. In 1941 the drug was dropped from the US pharmacopoeia (list of approved drugs), enabling pharmaceutical companies to similarly profit from alternatives. Dissenting official voices were still heard - the US Military Surgeon magazine declared that smoking cannabis is no more harmful than smoking tobacco, and in 1944 the New York Academy of Medicine reported that cannabis use does not cause violent behaviour, provoke insanity, lead to addiction or promote opiate usage. Anslinger described the authors as dangerous and strange.
In the same year, the New York Mayor's La Guardia Report The Marijuana problem in the City of New York similarly concluded that smoking marijuana did not lead to addiction in the medical sense of the word, that juvenile delinquency was not associated with marijuana smoking and that the publicity concerning the catastrophic effects of marijuana smoking in New York was unfounded. With characteristic tolerance, Anslinger then threatened doctors who carry out cannabis research with imprisonment. By 1948 this pleasant person had changed his message in one way, saying that cannabis users were peaceful rather than violent, but this did not change his opinion - he now stated that cannabis could be used during a communist invasion, to weaken American will to fight. The first UK cannabis arrest occurred in 1951, and the prospects for any immediate change in the law looked bleak indeed.