It is rather remarkable that the last-named four should all have flourished within so short a space of time. Chʽao Kung-wu accounts for it by saying: “During the early years of the Sung dynasty the Empire enjoyed a long spell of peace, and men ceased to practice the art of war. But when [ Chao ] Yüan-hao’s rebellion came [1038⁠–⁠42] and the frontier generals were defeated time after time, the Court made strenuous inquiry for men skilled in war, and military topics became the vogue amongst all the high officials. Hence it is that the commentators of Sun Tzǔ in our dynasty belong mainly to that period.” 113