The Spanish Empire was unique – without precedent or parallel in its day. There were maritime empires that tried to control trade and there were land empires that tried to control production. Only the Spanish Empire did both on such a vast scale: from Manila to Milan and from the Missouri to the Strait of Magellan. It swallowed almost in a single gulp the two states that were probably the most environmentally diverse in the early sixteenth-century world: those of the Aztecs and Incas. Spain spread commerce, contagion and cultural exchange across continents and oceans by routes never before traversed. Later, the British and French Empires registered similarly stupendous, similarly horrendous achievements on an even bigger scale. But they had the benefit of the Industrial Revolution behind them: longitude-finding mechanisms, electric telegraphy, rifled guns, steel cannon, steamships and railways, mass-produced remedies for tropical diseases and kit for tropical climes. By the time even the earliest of these advantages became available, the Spanish Empire was nearing its greatest extent. It was the first great world empire of land and sea to be constructed with pre-industrial technology.
So it is not surprising that, although for fifty years historians of empires have agreed that the best way to study their subjects is to compare them, no one has made more than selective and tentative efforts towards a comparative approach to the