Chinese officials boarded a small trading ship called the Arrow at Guangzhou on October 8, 1856, and took off twelve of its crew on suspicion of piracy. The ship was Chinese-owned and its crew were Chinese, but it flew the British flag on the strength of a registration that had expired three weeks earlier. Harry Parkes, the British consul at Guangzhou, demanded the men back and an apology. The imperial commissioner Ye Mingchen returned the men and not the apology. Britain attacked the city, and the Second Opium War began.
The pretext was thin. The Arrow's registration had lapsed, so under Britain's own rules the ship had no claim to the flag, and the men taken off it had been accused of a real crime. Parkes reported to Hong Kong that an insult had been offered to the flag, and the governor there, John Bowring, had been looking for an opening for some time: the opening of Guangzhou to foreign residents, compensation for earlier disputes, and revision of the treaties signed after the war of 1839 to 1842.
What Britain wanted was access. Trade at Guangzhou had run through a licensed monopoly of Chinese merchants for a century, and foreign traders had been kept outside the city walls, dealing only with the hong. The first war had opened five ports, but the interior, the rivers, and the right to send ministers to Beijing were all still closed, and the British government had been told by its merchants for a decade that the real prize was the whole country rather than a handful of coastal warehouses. France joined in 1857 after a French missionary was killed in Guangxi, and an Anglo-French force took Guangzhou that December and took Ye Mingchen with it.
The war ran four years. The Treaty of Tianjin in 1858 opened more ports, allowed foreign legations in Beijing and let missionaries travel inland. Fighting resumed, and in October 1860 British and French troops looted and burned the Old Summer Palace outside Beijing. The Convention of Beijing that followed ceded the Kowloon peninsula to Britain. Chinese historians class both treaties among the unequal treaties, and the Arrow is taught as the pretext for a war that had already been decided on.