I originally approached this topic from an interest in nomads. Reading up on China's relations with the Mongols in the summer of 1991, I was struck by the strategic dilemma facing the Chinese. The Chinese had to defend a thousand miles of frontier against an enemy with superior mobility, one who could choose the time and place of an attack almost at will. The Mongols lived in tents and traveled with their flocks, so the Chinese could not pin them down. Nor could the Chinese possibly hold every point along the frontier with sufficient strength. Arthur Waldron's The Great Wall of China analyzed the range of options open to the Chinese and showed that the decision to fortify the frontier in the 1500s was anything but a foregone conclusion.

My interest in the Mongols then took me to other places they had been, like Russia and Iran, and I was struck by the similarities and differences with the strategic situation in China. The Russian border defenses were similar in principle to those in China, albeit far less elaborate. On the other hand, there were no such defenses separating Iran from the steppe. Although China and Russia broke free of the Mongol empire in the 1300s and 1400s, respectively, Turks gained and maintained the upper hand throughout the Middle East and India. The Middle East and India might be taken as an illustration of what happened when the defenses failed, except there was no evidence of such defenses, and anyway they had failed in Russia and China without similar consequences.