Geography is a redoubtable enemy. It beat Napoleon in Russia, Alexander in India, Cambyses in the Sahara and America in Vietnam, where vast distances and an unfamiliar environment impeded logistics and enervated the troops. It sank much of the Spanish Armada and fended the Mongols from Japan. But geography isn't invincible. Cortés overcame it by enlisting native collaborators to conquer Mexico. A half-century later the Mughal emperor Akbar used technology to challenge it at the Rajput stronghold of Chittorgarh, cracking the mountaintop citadel with explosive mines. José San Martín relied on sheer...