To understand how cannabis came to be banned in Canada, we need to look at the history of what is now called the “War on Drugs.”
During the 1800s, Britain’s naval supremacy allowed its merchants to dominate the global trade in profitable drugs like alcohol, tobacco, opium and tea.
The British people were very heavy users of tea leaves imported from China.
Britain didn’t have anything of interest to the Chinese, except silver. The trade imbalance was bankrupting the British treasury.
The British East India Company had a monopoly on India’s opium supply, and they began selling opium to the Chinese market. Opium was popular among the Chinese, but as its use spread the Chinese government tried to ban it. Prohibiting opium only served to keep prices high, ensuring greater profits for British smugglers.
• Britain's naval supremacy
Wikipedia: History of the Royal Navy, French Revolutionary and Napoleonic Wars (1793–1815)
• except silver
Article: Selling Opium to China to Pay For Tea, by Claire Petras, British Tea, 17th-19th Century
• trade imbalance, opium ban
Article: The First Opium War (1838-1842), by Vern Cleary, Modern World History, Online textbook, Bellarmine College, 2013