“From the 1970s on, the United States would seem less and less united with each passing decade” goes the thesis of Fault Lines, the new history from Kevin M. Kruse and Julian E. Zelizer. Based on the authors’ class at Princeton, Fault Lines: A History of the United States Since 1974 reiterates that premise a lot over its 400 pages, but its deep detail and taut-as-a-thriller pacing make up for the repetition.

Beginning with Watergate, the authors detail how the Democratic and — especially — Republican parties moved the country from post-New Deal liberalism to an increasingly hard-right philosophy, culminating with Trump. They demonstrate the ways that the 45 playbook resembles Ronald Reagan’s, notably by staffing social services with their sworn enemies and aggressively courting the religious right. (The Reagan-approved end of the “fairness doctrine” in 1987, which ushered in an era of partisan TV and radio coverage, comes in for blame as well.)

The authors are careful to note how often both Presidents Clinton and Obama tacked to the right of their own bases, helping push the GOP even further. September 11th, too, is handled deftly: We see George W. Bush move from “compassionate conservatism” to telling an aide, “Fuck Saddam. We’re taking him out.” If Fault Lines doesn’t provide easy answers to our current dilemma, its cleareyed, pin-sharp overview is a necessary map of how we got here.