Nations have historically sought power and prosperity through control of physical space. In recent decades, however, this has largely ceased. Most states that could do so appear reluctant, while the weak cannot expand. This article presents a theory of imperialism and decolonization that explains both historic cycles of expansion and decline and the collective demise of the urge to colonize. Technological shocks enable expansion, while rising labour costs and the dynamics of military technology gradually dilute imperial advantage. Simultaneously, economic development leads to a secular decline in payoffs for appropriating land, minerals and capital. Once conquest no longer pays great powers, the systemic imperative to integrate production vertically also becomes archaic.