Given the task definition and input, reply with output. This task is about reading the given passage and construct a question about the information present in the passage. Construct a question in such a way that (i) it is unambiguous, (ii) it is answerable from the passage, (iii) its answer is unique (iv) its answer is a continuous text span from the paragraph. Avoid creating questions that (i) can be answered correctly without actually understanding the paragraph and (ii) uses same words or phrases given in the passage.

Ratification of the treaty the following year led to resumption of hostilities and in 1860, with Anglo-French forces marching on Beijing, the emperor and his court fled the capital for the imperial hunting lodge at Rehe. Once in Beijing, the Anglo-French forces looted the Old Summer Palace, and in an act of revenge for the arrest of several Englishmen, burnt it to the ground. Prince Gong, a younger half-brother of the emperor, who had been left as his brother's proxy in the capital, was forced to sign the Convention of Beijing. Meanwhile, the humiliated emperor died the following year at Rehe.
Whed did the British and French invade Beijing?