The police response to protests erupting on America's streets in recent years has made the militarization of policing painfully transparent. Yet, properly demilitarizing the police requires a deeper understanding of its historical development, causes, and social logics. Policing Empires offers a postcolonial historical sociology of police militarization in Britain and the United States to aid that effort. Julian Go tracks when, why, and how British and US police departments have adopted military tactics, tools, and technologies for domestic use. Go reveals that police militarization has occurred since the very founding of modern policing in the nineteenth century into the present, and that it is an effect of the "imperial boomerang." Policing Empires thereby unlocks the dirty secret of police militarization: Police have brought imperial practices home to militarize themselves in response to perceived racialized threats from minority and immigrant populations.