The Mohammedan Conquest of India is probably the bloodiest story in history. It is a discouraging tale, for its evident moral is that civilization is a precarious thing, whose delicate complex of order and liberty, culture and peace may at any time be overthrown by barbarians invading from without or multiplying within. The Hindus ……had failed to organize their forces for the protection of their frontiers and their capitals, their wealth and their freedom, from the hordes of Scythians, Huns, Afghans and Turks hovering about India's boundaries and waiting for national weakness to let them in. For four hundred years (600-1000 A.D.) India invited conquest; and at last it came….. The bitter lesson that may be drawn from this tragedy is that eternal vigilance is the price of civilization. A nation must love peace, but keep its powder dry.
-- Will Durant, Our Oriental Heritage, 1930