Historically, the expansion of the American frontier symbolized a unity between political liberty and economic growth, at the same time as it justified the violent expropriation that continues to define the country's racial and distributional politics.

As commentators and policymakers have scrambled to find explanations for and responses to the unprecedented uprisings against police brutality across the United States, interest in the role of police unions in local politics has soared. Recent research into the question…