I watched a documentary once about the Great Wall of China. In it, it claimed that a soldier or guard at the time of one of the dynasties let an enemy force through in exchange for food because he was starving due to not being paid enough. I am writing a white paper on physical security and wanted to reference this as a fact but am finding it hard to search for this because of all the Chinese restaurants called Great Wall and most of what I find on protecting the wall is about protecting it from deterioration in modern times.
Can someone confirm this as fact with more details to help me search for references (ie. Which dynasty, which enemy, etc).