Nations have historically sought power and prosperity through control of physical space. In recent decades, however, territorial empire has largely ceased. Most states that can take and hold territory no longer appear eager to do so, while the weak are unable to expand. Have powerful countries become 'kinder and gentler', or has something fundamental changed about the logic of empire? We offer a theory of imperialism and decolonization that explains both historic cycles of expansion and decline and the demise of the urge to colonize. Technological shocks enable expansion, while military technology gradually disseminates, diluting imperial advantage. At the same time, economic development has led to a secular decline in the payoffs for appropriating land, minerals, and reluctant labor. Once conquest no longer pays for great powers, the systemic imperative to vertically integrate production also becomes archaic.