In 1833 the East India Company would lose its monopoly on Chinese trade. This change, and the lack of an effective structure to replace it, would create a power vacuum in and around China's only western trading port (Canton).

British Opium Traders took this opportunity to flood in and make fortunes, and to use those fortunes to promote a more belligerent diplomatic stance towards China. 

As the Qing Empire attempted to stamp out the Opium trade, Canton, now awash with Opium, became the flashpoint for a conflict neither the British Empire, nor the Qing Empire actually wanted.

Read more on the Opium War:

Imperial Twilight: The Opium War and the End of China's Last Golden Age - Stephen Platt
The Opium War: Drugs, Dreams, and the Making of Modern China - Julie Lovell