An unhappy combination of imperialistic greed, horrid diplomacy, cultural misunderstanding, and extreme xenophobia were to blame. At the time leading up to the conflict, the UK had been seeking a permanent embassy mission in Beijing, something the Qin dynasty would have none of. In the eyes of the Qin, China was not a state among states. Rather, she was the center of all civilization. Her rule, celestial and above all that of lesser men. Any foreign diplomat was in fact paying tribute, and was there to bask in the glory and earn the favor of the Celestial Court. As you can imagine, Britain didn't really see it the same way. After a series of attempts to open relations, British diplomats were consistently condescended and dismissed. In one poignant example, a British envoy was asked to kowtow (fall on one's knees and bow) to a letter written by the emperor. During this time, opium was being traded in the southern regions by British merchants seeking to find something that China would trade for. At first welcomed by the Qin regime (they had their share of tax revenue from it), the trade had become a major social problem. In this tense setting, several incidents occurred at both a public and private level. In one major incident, drunken British sailors beat a Chinese national to death, and the failure of the British government to extradite the responsible party further enraged Chinese officials. A military escalation seemed bound to happen. China sent in war ships to Kowloon. British naval forces fired a warning shot at a private English ship, and the situation erupted into armed confrontation. The Chinese ships were outclassed, outgunned, out trained. The rest of the war unfolded into a century of disaster for the Chinese people.

1 correction, Qing (not Qin) China, maintained a "canton system" of trading posts in Southern China specifically for foreign trade. The amount of foreign trade was actually more substantial (including imports) than most have stated.

Granted, with the British, the British had a huge trade deficit with China, Largely because of British "love" for Tea, which China had a monopoly for a long time.

The British began to smuggle Opium into China to balance the trade. Opium was illegal by Qing imperial edict since before 1799.

When the Chinese commissioner Lin Zexu seized and destroyed smuggled Opium (in "international waters"), the British traders accused him of "destroying their property", and lobbied the British government for war.