Context: https://en.wikipedia.org/wiki/First_Opium_War
I heard linking Wikipedia is hard to do for this format. Bonus meme:
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There was also tea at stake.
China enjoyed a trade surplus with Europe, trading porcelain, silk, and tea in exchange for silver. Trade between China and Britain was heavily imbalanced in China's favor. As Britain's demands for tea, silk, and porcelain exceeded Chinese demands for British goods, British merchants started exporting opium in India. This shift reversed the balance of trade.