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From its base in India, the Company had also been engaged in an increasingly profitable opium export trade to China since the 1730s. This trade, illegal since it was outlawed by the Qing dynasty in 1729, helped reverse the trade imbalances resulting from the British imports of tea, which saw large outflows of silver from Britain to China. In 1839, the confiscation by the Chinese authorities at Canton of 20,000 chests of opium led Britain to attack China in the First Opium War, and resulted in the seizure by Britain of Hong Kong Island, at that time a minor settlement.

What illegal trade was the British East India Company involved in?
opium