When did Great Britain and France declare war on China?

In the mid-1850’s, the United States and Great Britain sought to renegotiate their commercial treaties with China as Britain requested a series of allowances: full access to China for their merchants, an ambassador in Beijing, legalisation of the opium trade, and the exemption of imports from tariffs. However, unwilling to make further concessions to the West, the Qing government of Emperor Xianfeng refused.

On 3rd of March, 1857, British forces arrived at Hong Kong, who joined with the French and declared war, attacking the forts on the Pearl River south of Canton. The governor of Guangdong and Guangxi provinces, Ye Mingchen, ordered his soldiers not to resist and the British easily took control of the forts. Advancing north, the British and French seized Canton after a brief fight and captured Ye Mingchen, before sailing north and taking the Taku Forts outside Tianjin in May 1858. Xianfeng was unable to resist the advancing British and French, so in an attempt to seek peace, the Chinese negotiated the Treaties of Tianjin. As part of the treaties, the British, French, Americans, and Russians were permitted to install legations in Beijing, ten additional ports would be opened to foreign trade, foreigners would be permitted to travel through the interior, and reparations would be paid to Britain and France.