On October 23, 1856, British warships attacked the Chinese forts on the Pearl river below Guangzhou, and the Second Opium War began. The immediate cause was an incident earlier in the month, when Chinese officers boarded a lorcha called the Arrow at Canton and arrested part of its crew on suspicion of piracy and smuggling. The vessel was registered in Hong Kong, and the British consul, Harry Parkes, demanded an apology and the release of the men, and the demand was refused. The flag became the argument.
The background was wider than the ship. Britain, France and other powers wanted more ports open to trade, the right to send envoys to Beijing, and a revision of the treaties of 1842 and 1844 that had ended the first war, and their requests had been turned down. The chance came. British forces took the forts, then entered Guangzhou, and in 1857 they were joined by France, which had its own grievance over the killing of a missionary in Guangxi.
The allies took Guangzhou in December 1857 and later moved north, and they took the Dagu forts outside Tianjin in 1858 and forced the treaty of Tianjin on the Qing court, and in 1860, after the war resumed, they entered Beijing and burned the imperial summer palace. The terms were hard. The treaties of Tianjin and Beijing imposed on China opened more ports to foreign trade, allowed foreign envoys in the capital, gave protection to Christian missions, and gave Britain the Kowloon peninsula opposite Hong Kong island.
The wars of the 1850s and 1860s left China with less control over its own trade and frontiers, and the terms imposed at that time were among the unequal treaties that Chinese governments worked to revise or end through the following century. The Kowloon peninsula was restored to China together with the New Territories in 1997, and the events of the period are part of the teaching of modern Chinese history.