Unluckily for the British, the first opium war had failed to remove the ban of the drug in China.
By 1854, British imports that came from China had reached nine times more than they were in past years. British officials became under pressure and only opium could balance the trade, so they had to keep trading the illegal drug with Chinese merchants.
The second opium war was caused by the "Arrow incident." The arrow was a boat/vessel with an expired British trading registration which was seized by the Qing dynasty for the smuggling of salt. The British complained to the high ranking generals and officials of the Qing dynast, more specifically, Ye Mingchen, that the Chinese seizing The Arrow violated a Chinese law. The matters developed and the second opium war broke out. It lasted from the 8th of October in 1856 and lasted until the 18th of October in 1860, (4 years.) This war took place in China.
A painting of the second opium war