The British had interest in Opium in the Opium Wars or Anglo-Chinese Wars. Actually, that’s not the entire truth.
They didn’t want the opium per se, they wanted the freedom to sell their opium to millions of Chinese.
That’s the great thing about addicts: They make good customers. The British weren’t the only ones to profit from the opium trade: the Americans also did, as well as the Turks and Indians.
China blockaded the British from dealing opium in Canton. The Chinese government had grown weary of watching westerners turn a profit by addicting their countrymen to the potent (and then illegal) drug.
They seized all of the British opium in 1839 from Cantonese warehouses, refused to apologize, forbade all trade with Britain, then fired on British warships in their waters. Well, British honor wouldn’t allow this effrontery. In retaliation, the British seized Hong Kong and bombed Canton.
Finally, China agreed to pay reparations, gave Hong Kong to the British, and reopened several ports to British trade again. This 1842 agreement was called the Treaty of Nanking.
As for the British side of the deal, they got to continue to addict millions more of the Chinese, rake in the profits, and have access and trade with China at no extra cost.
As a result, other western countries followed suit, securing similar open and free rights to residence and trade within China’s borders.
This time period is referred to by the Chinese as “the time of unequal treaties,” as they continuously lost control over their own country to stronger western countries like Great Britain, France, Russia, and the United States.