With a handful of exceptions, no country in world history has incarcerated as large a share of its population as does the contemporary United States. Yet while most research on punishment observes this comparative and historical fact, historical and especially comparative research into mass incarceration is rare. Most research has been America-centric and presentist: that is, it has built explanations for the American punitive turn by studying America over the period of the punitive turn. We use original data from the early 19th century to the present across more than a hundred countries to illustrate descriptive and explanatory issues that have gone mostly unnoticed in existing work. Some simple decompositions suggest that American mass incarceration is the result of America's unique combination of strong but uneven state capacity and high levels of violence. We use a triplet of paired comparisons (the US to Canada and Australia, the US to Brazil and South Africa, and the US to the USSR) to suggest that this combination of facts has its roots in American slavery, though not in ways commonly supposed. We propose that American slavery matters to mass incarceration not because of its cultural or ideological legacy, but because of the role it played in deforming American political and economic development.