The First of the Opium wars started in 1839 when the Daoguang Emperor rejected proposals to legalize opium trade and impose taxes on it. He subsequently orders to cut supplies and confine foreign traders who were caught involved in these activities.
As a response, Great Britain decided to engage in war with China. As they considered that the Chinese were inducing the death of British traders by cutting them their supplies.
After the British Victory, the Chinese were forced to sign what their people consider nowadays as the "Unequal Treaties" that triggered the start of the "Century of Humiliation". Hong Kong was transferred to the British for their administration and the Chinese lost all tariff autonomy over their goods.