How did western powers control trade with china

The opium wars forced the Chinese to open Chinese ports to western governments and made the trade in opuim legal.

Western powers controlled trade with China through the Unequal Treaties.

Explanation:

China and some foreign powers signed the first unequal treaty at the beginning of the First Opium War (1839-1842). After the subsequent defeat of the Qing, China had to sign more treaties with the British, who forced them to open several of their ports to foreign trade, while allowing Christians to reside and evangelize freely.

The term "unequal treaty" was not used until the beginning of the 20th century, although many Chinese people noted previously the inequality in the terms of the treaties. In many cases, China was effectively forced to pay burdensome war indemnities, to open ports for trade, to cede territories (such as Hong Kong and the Kowloon Peninsula that fell into the hands of Great Britain) and also to make several concessions of sovereignty to foreigners.

The Chinese Unequal Treaties prescribed almost completely only after the delivery of Hong Kong in 1997 (promised in 1984, which allowed the dialogue between Deng Xiaoping and the British) with the exception of Macau and the territories occupied by the Russian Empire (Foreign Manchuria).