The initial trade was only for purposes of opening up China for global trade, and to create vulnerability to exploit and colonise, like neighbour India was colonised for 200 years. Britishers consistently stated the need for “laissez faire” trade for a reason.
The reason why a second opium war happened was that the first one was merely lost, and the 8 Nation Alliance failed to fully colonise. Infact, some ports and provinces of China were given/gifted to European powers during this time, and withdrawn back after the wars.