The history of narcotics prohibition is essentially the history of American narcotics prohibition, and then their success in internationalizing it. At the turn of the century, you could buy cocaine, morphine, or heroin over the counter at most any pharmacy.
The origins of prohibition begin in 1898 after the US government inherited an opium monopoly in the Philippines following the Spanish-American war. The Spanish colonial state had secured and maintained control of the opium trade, and promoted the use of opium among the population at state-licensed stores. The US now controlled this profitable monopoly.
While initially, American authorities planned to continue the trade in order to shore up the colonies' finances, missionaries coming from the US were appalled by the widespread use of opium, and the very idea that the government would actively profit off it was abhorrent to them. Reverend Wilbur Crafts led a successful religious campaign against the monopoly, and it was decided to continue selling opium for three years in order to 'ween off' users before full prohibition in the Philippines.
It is important to recognize that this prohibition hadn't, by this point, spread to the US. Many Americans were still buying opium-derived products from their local pharmacy without any serious regulation. The reason it was considered morally reprehensible in the Philippines but not the US is that 1) people in the Philippines smoked opium, which was considered more harmful because of its links to the Yellow Peril panic, and 2) paternalistic ideas of a 'white man's burden' justified a more controlling approach to non-whites. Local panics had already motivated city and state-level restrictions on the sale of smoking opium on the west coast. At almost every stage of prohibition, racial factors were key.
Prohibition in the Philippines quickly ran into problems because the US was unable to prevent large scale smuggling of opium into the colony. The trade was a massive industry during this period across East Asia. US policy-makers decided that prohibition in the Philippines would only work if it was done as part of an international agreement. This played into geopolitical maneuvering by the US because the other major power pushing for international prohibition was China and President Roosevelt was pursuing a closer relationship with the country.
Thus at the 1909 Shanghai Conference, the US and China were able to achieve a non-binding agreement with the other colonial powers, most notably Britain, to limit the opium trade. American negotiators complained that they weren't able to push further because the US lacked any federal prohibition of narcotics. As a show of good faith, the US began prohibiting the import of smoking opium from April 1909. Attempts to go further were blocked by the pharmaceutical industry. After several more years of negotiating, the first binding international drug control treaty, the International Opium Convention was agreed upon in 1912.
The battle between congressional allies of the pharmaceutical industry and moralizers escalated in the early 1910s. Moralizers wanted firmer domestic restrictions but were repeatedly blocked. In order to circumvent the congressional deadlock, moralizers allied themselves with the newly ascendant Southern Democrats by playing into a racial panic about black cocaine usage. Cocaine had been taken widely by white society for many years, but fears began when use spread to minorities. Initially, black usage stemmed from employers who gave cocaine to black workers in order to give them more energy to work. When usage spread from there, city authorities employed a policy of 'containment' whereby selling of cocaine was pushed into poorer areas. By the 1910s this had caused cocaine use to be associated with blackness.
By allying with Southern Democrats, and Western representatives concerned about the Yellow Peril, the moralizers were finally able to pass the Harrison Act in 1914. This made it illegal to sell all opium-derived products, as well as cocaine, without a prescription. The Act was passed as a tax-law, in order to circumvent constitutional restrictions on federal power.
Just to highlight the role of racial politics in passing the Harrision Act, here are two of the arguments made by US Opium Commissioner Dr. Hamilton Wright supporting the need for federal restrictions: "it has been authoritatively stated that cocaine is often the direct incentive to the crime of rape by the negroes of the South and other sections of the country". And: "one of the most unfortunate phases of smoking opium in this country is the large number of women who have become involved and were living as common-law wives or cohabitating with Chinese in the Chinatowns of our various cities"
This was the watershed moment for drug prohibition. Over the following decades, moralizers used the same coalition to tighten restrictions. Of particular note is Harry Anslinger who headed the Treasury Commission for the Federal Bureau of Narcotics. His personal campaign against marijuana resulted in its prohibition in 1937.
He's a fascinating figure because he essentially manufactured a moral panic surrounding marijuana use by communists, African American's and Mexicans, out of thin air. He sent press releases to newspapers detailing the dangers of marijuana, in which he invented crimes, and then used the resulting newspaper stories in congress as evidence.
The most ridiculous of these was a series of articles called the 'Gore Files' which graphically described 200 marijuana motivated violent crimes by minorities - but subsequent research found 198 of these crimes did not, in fact, involve marijuana, and the remaining two never happened at all.
His publications were deeply racialized, including: "Two Negros took a girl fourteen years old and kept her for two days under the influence of hemp. Upon recovery, she was found to be suffering from syphilis" and "Reefer makes darkies think they're as good as white men."
/u/fevercrumb1649 provides a good overview of the earlier period of narcotics prohibition in America, and /u/SomeoneGMForMe’s also gone into more detail into the racial dimensions. However, given the international scale of OP’s question, in my answer I’d like to emphasise three different questions surrounding the internationalization of prohibition that differ from the existing comment.
Drug control has a long and contested history. The prohibition of opium under the Qing dynasty is a clear instance of an international dispute surrounding the ban of drugs for moral, economic, political, diplomatic reasons; five centuries prior to the American ban, Siam had already done so. Nonetheless, as Marx observed, opium’s profitability meant various merchants, governments, and even bankers were dependent on the ‘free trade in poison’. Close to one seventh of British India, and even China’s income stemmed from the opium monopoly in the last two decades of the 20th century: states were very much the origin, rather than suppressor, of widespread drug economies.
How and why did this change? One argument was that a global prohibitionist drug control regime was predominantly the effort and preference of the US and Europe: Ethan Nadelmann argues that Asian states could have constructed a different regime in the early 20th century permitting opium, and the disappearance of this ‘path not taken’ reflects largely the “desire and capacity of the United States to impose its drug-related norms on the rest of the world”. (Nadelmann 2006: 511) In light of a highly visible war on drugs, this narrative is a tempting one, but one underplaying the influence that various other countries, organisations, and even individuals played. Even as it is true that the US has drafted and lobbied for virtually every major drug control convention, and shaped not just the principle but form of drug control by exporting funds and enforcement strategies. As James Windle argues, even as the global regime is anchored on an ‘American prohibitive model’, “this is not the same as prohibition being an American or Western construct”. (Windle 2013: 1194
The first international conference on narcotics would indeed have been in Shanghai in 1909, as fevercrumb observes, which facilitated the 1912 International Opium Convention of the Hague (which entered in force in 1915). The Treaty of Versailles contained a clause mandating adherence from its signatories to the Opium Convention, marking the start of something resembling the international instruments of today. It was the League of Nations in 1920 that assumed responsibilities for drug control, developing 3 major conventions (1925, 1931, and 1936) that had provisions to curb the drug trade in practice.
The now forgotten 1936 Convention for the Suppression of the Illicit Traffic in Dangerous Drugs largely failed due to a lack of state support. WWII’s outbreak did sap away state support, but it was primarily the treaty’s intervention into domestic criminal law, perceived by many LON members as key to state sovereignty, that led to its failure. As British lawyer J.G Starke noted, the convention’s treaties were ambitious in areas like criminalization, extraterritorial jurisdiction, extradition, and legal assistance. Given that these were successfully adopted only in the later 1988 Drug Trafficking Convention, the acceptance of drug treaties required wider shifts in international law and the rethinking of sovereignty in the post-war period.
The experience of East Asia provides us a valuable detour into an oft-overlooked history of drug control. First: on Imperial Japan’s own ambiguous approach towards drug markets. In the 1920s and 1930s, it experimented with legalizing and regulating drugs like opium within its colonies in Taiwan and Korea, as well as in occupied Chinese territory (for more detail, see Jennings 1997). Despite opium being characterized at the Tokyo International Military Tribunal as an exploitative commodity employed by Japanese administrators to extract value and income from its conquered societies, Japanese policy often oscillated between periods of reform and exploitation – its policy in Formosa/Taiwan of gradual suppression was widely discussed even by the 1909 Opium Commission.
At the same time, Chinese drug policies evoked complex sentiments in European societies: the image of an opium den brought to mind a ‘reverse colonisation’. Imperial guilt, the dread of decline, and the threat of reprisal featured in popular literature (Wilkie Collins, Arthur Conan Doyle), expressed by figures like Malcolm Delevigne (the chief British diplomat to the LON on drug policy). In 1935, he opined that following the British cultivation of Indian opium to force into Chinese markets, the ghosts of those Chinese addicts threatened residents of London, Manchester, and Birmingham. “The tables will indeed be turned with a vengeance if the Far East, which has been one of the chief victims of the illicit traffic from the West, should now, armed with the knowledge that the West has taught it, become a menace to the West itself”. Drug control was an imperative triggered by imperially-tinged associations of guilt, fear, revulsion, anxiety.