The First Opium War started because historically, Great Britain imported a lot of tea from China but had struggles with finding British products that the Chinese would buy from them. This caused trade imbalance.
Because of that around the mid-18th century, the East India Company started to sell opium to British traders, who shipped it to China, even with the fact that China banned it.
With time, this trade started to increase addiction among Chinese citizens and increased social problems. With that, the Chinese government confiscated and destroyed opium that was brought there by British merchants. Tension rose, the British destroyed a Chinese blockade at Hong Kong.