Heather Hurlburt directs the New Models of Policy Change project at New America. She held foreign policy positions in the White House, the State Department and Congress under Republican and Democratic administrations.

In “The Great Rift,” James Mann asserts that the George W. Bush administration’s internal struggle over whether and how to go to war in Iraq was “the peak in acrimony in the running philosophical debate about America’s post-Cold War role.” Some might disagree, including the professionals who testified on Ukraine in congressional impeachment hearings last month, or the former Cabinet members who sought to convince President Trump of the value of alliances in Europe and Asia. But Mann’s premise is that the debate over Iraq, as seen through the lens of the 40-year relationship between Colin Powell and Dick Cheney, can help us explain those vexed debates in our own moment:

How and why did the country tumble from such optimism at the time of the Persian Gulf War and the collapse of the Soviet Union to the divisions, uncertainty and diminished confidence that prevailed two decades later?

Mann has a distinguished oeuvre chronicling the heights of the U.S. foreign policy establishment, particularly its Republican subculture and its attempts to reinvent itself after the Cold War. His method, perhaps used to greatest effect in “Rise of the Vulcans,” his study of the first-term team around Bush, is to look intently at the relationship dynamics among senior players. He brings all his prior knowledge to bear on the rise and decline of Powell’s and Cheney’s careers — and their friendship.

In Mann’s telling, the two men meet cute during the Reagan administration, when Lt. Gen. Powell briefs Rep. Cheney on a visit to a U.S. base in Germany, and the men immediately recognize each other’s brilliance. Though Mann never says so, perhaps each sees in the other a slight distancing from the security establishment in which they swim, a coolness in appraising it and a willingness to use it to their advantage. After George H.W. Bush is elected president in 1988, Cheney moves from Congress to the Pentagon as secretary of defense, and within a year, Powell joins him there as chairman of the Joint Chiefs of Staff. Even then, the two are frequently on different sides, disagreeing on Mikhail Gorbachev, Iraq’s invasion of Kuwait, the breakup of the Soviet Union.

Cheney and his team spend the H.W. years developing a deeply unilateral view of American power, one that Powell goes to some lengths to oppose while still in government. When, during the Clinton years, Cheney is one of 25 signers of the Project for a New American Century’s founding call for a more assertive U.S. foreign policy, Powell is not even approached about signing.

And so we arrive at the George W. Bush administration. Although Powell and Cheney still saw each other as friends, they quickly found themselves at the head of opposing Cabinet factions. Even before 9/11, their relationship had become acrimonious, as Cheney used his position in the White House to outmaneuver Powell’s State Department on policy issues. Meanwhile, Powell’s star power with the American public discomfitted colleagues and conservative activists outside government.

Mann’s retelling is a useful summary for those who didn’t live through the period at close range, and it offers some new nuances for those who did. His portrayal of Cheney stresses his skill at using bureaucracy and proximity to get his way, as well as his focus on unilateral U.S. power. Powell, while a far more adept public figure than Cheney, falls just short of him in bureaucratic skills and is bedeviled, in Mann’s telling, by a lack of strategic vision and decisiveness. When Powell is made aware of the decision to torture detainees, he does not speak out — and does not tell his staff. Ultimately, when he has the chance to tell Bush that he will not support an Iraq invasion, he does not do so. Instead, he delivers the administration’s justification for war on the world stage — based, as it turns out, on incorrect intelligence pushed forward by Cheney.

Mann wants his readers to know that Cheney had no post-9/11 change of heart. He documents the consistency of his deep conservatism and unilateralism across decades. Mann is less clear in explaining what became of the GOP foreign policy establishment’s talent for teamwork and realism. He seems to believe that Cheney’s unilateralism and drive for power helped provoke the fatal errors of the Iraq War, while Powell’s “passivity” prevented a meaningful challenge to the war from taking shape inside the administration. Or that is my interpretation — Mann never offers a judgment.

He walks us through the errors, misjudgments and flat-out untruths that got the United States into Iraq but can never bring himself to answer his own question of why. And though he flirts in the volume’s opening and close with an even larger question — how did the establishment that presided over the end of the Cold War and the triumphal Gulf War find itself either handmaidens or ineffectual opponents of President Trump? — he offers no answers there, either.

Mann writes with a curious indifference to the national security debates of the moment. He notes in passing Powell’s disenchantment with the GOP even before the end of the Bush administration, as he was pushed away by the blend of unilateralism in foreign policy and anti-immigrant and anti-Muslim sentiment at home. And Mann dissects carefully how Cheney’s focus on power over values differentiated him from the neoconservatives of the Bush era, who believed they could use force to promote democracy, tolerance and pluralism — while Cheney saw rights and norms as beside the point of security affairs.

We now know that those cleavages prefigure rather excruciatingly the dismemberment of the Republican national security establishment into Never Trump and Trump-accepting wings. We also know that the Islamophobia that discomfits Powell throughout the post-9/11 years moves steadily into the mainstream of the party, with some of its key spokespeople now holding jobs in Mike Pompeo’s State Department. Cheney, it should be noted, diverges profoundly from Trump on matters including trade policy, respect for the institutions of the U.S. military and attitudes toward dictators such as Kim Jong Un.

Perhaps, though, Mann isn’t able to shed more light on this because the bottom line on how we got here is less about the strengths and weaknesses of individual foreign policy mandarins and more about how our national security choices interacted with our domestic politics. The story he tells unfolds over decades that saw the GOP pick up anti-affirmative-action rhetoric, Islamophobia and anti-immigrant sentiment as key campaign platforms. Those developments could have featured in this book, but they don’t.

Similarly, Mann quotes Powell himself on how the legacies of race in America shaped his life: the perception that his rise was due to affirmative action, his need to make money after government service, even his acceptance by the public because (in his words) “I ain’t that black.” But Mann never steps back to consider the role race played not just in Powell’s rise to national prominence but in his fall from conservative favor, even as he repeats a critique of Powell — that he is an action officer, not a strategic thinker — that professionals from minority and nontraditional backgrounds have heard for generations. His ascendance in the GOP and then alienation from it would seem to tell a powerful story about millions of Americans’ conflicted relationship with the party of Lincoln, and the discomfort of millions of others with an America where, for a time, the idea that Powell, as a Republican, could have been the first African American president was entirely plausible.

But to do that, Mann would have had to weave in more of the world outside the triangle formed by C Street, the White House and the Pentagon. His choice not to do so limits the explanatory capacities of this careful book. And it stands as a reminder that the forces that have the upper hand in the Republican Party have not hesitated to turn economic and cultural concerns into “national security issues” — a political strategy for which neither Mann’s old GOP establishment nor its Democratic counterparts seem to have an answer.

The Great Rift

Dick Cheney, Colin Powell, and the Broken Friendship That Defined an Era

By James Mann

Henry Holt. 416 pp. $32