长江天堑

Explanation: Cutting: trenches. Pit cutting the Yangtze River as a natural, strategic importance. Old described the difficult terrain of the Yangtze, insurmountable. 　　Source: "Appreciative Sun Fan Biography" Sui division will be economic Jiang, Qun Guan Please prepare defense, Fan said, playing 'the Yangtze River natural moat, separated since ancient times insurance, how can the military Krupp Stagecoach? '" 　　Examples: Occupy the southern eight counties across the ~, two much worse than Huaixi come and go. (Ming Shinai "Tales of" one hundred and back)