Torching of the Summer Palace is an event which happens to 
China once a Great power enacts the decision Burn the Summer Palace. The decision is possible to take for a great power, if it is at war with China, and have occupied Beijing. It costs the great power 3 infamy to take.
It has a single option We Weep for our loss which gives -25 prestige and 5 war exhaustion.
It is generally not worth it, for a great power to take, but that does not mean, that the AI cannot decide to do it anyway. For that reason, it is very important for China, never to let any great power occupy Beijing.