But it’s funny what a tweak to history can — and can’t — do, an idea Ackerman explores thoughtfully, if at times a little dryly. (Again: President Al Gore.) It’s 2004, and America is largely untroubled by overseas crises. After Bill Clinton was convicted following his impeachment trial, Gore rose to the Oval Office, then handily defeated George W. Bush in 2000. The twin towers were still toppled on 9/11, but America focused its response on Afghanistan, and Osama bin Laden was dead by Christmas. “Would Bush have possessed Gore’s forbearance in the Middle East?” muses the novel’s narrator, Martin. “I’ve often wondered.”

Martin is a Civil War historian, working on a book in the guesthouse of Robert Ableson, a wealthy, semiretired lawyer, near Richmond. Martin’s working thesis, borrowed from the historian Shelby Foote, is a centrist crusade to validate “the role of compromise in the sustainment of American life, as well as our relatively recent departure from it as an American virtue.” That “departure” refers to a rise in divisive politics, especially around social justice issues. Front and center in “Halcyon” is the matter of Confederate monuments: A rival historian and an Ableson family acquaintance are leading a protest against the statue of Robert E. Lee at Gettysburg. Martin is a milquetoast on the matter, hand-waving about heritage and compromise. Ableson, for his part, is more firm in supporting the sculpture: “Why would a history professor want to destroy a piece of history?”

Ableson can dig in his heels on a counterprotest because of his wealth. (The novel’s title refers to the name of his estate.) He can also persist because, much like the vampires hunted by alternate Lincoln, he’s undead. Medical technology has discovered a way to resuscitate the recently passed, and Ableson is one of the early, secret beneficiaries. This complicates his will, naturally. But it also complicates history in general. “History is what the living think of the dead,” Foote writes in a book inscribed to Martin. So what becomes of history, Ackerman asks, if we all get to live forever?

Ackerman, as much as any working novelist today, is invested in getting the facts of war and history right. Before pursuing a writing career, he was a Marine officer who served five tours of duty in Iraq and Afghanistan, and he’s channeled the tragedies of U.S. military adventures into fiction and nonfiction, including the novel “Dark at the Crossing,” which was a National Book Award finalist. To write a story such as “Halcyon,” in which America avoids a 21st-century military quagmire, is certainly a case of partial wish fulfillment.

But Ackerman also seems to argue that different presidents than those we’ve had wouldn’t eradicate bad foreign wars or stateside divisiveness. The lack of a Middle East war in “Halcyon” essentially fast-forwards America’s domestic reckonings, moving public conversations around #MeToo and systemic racism to the fore. (Gore’s pardon of Clinton, to some, represents “exonerating a man for rape”; Ableson, now revived, is asked to answer for a colleague’s sexual abuse allegation.)

“Halcyon” is an entertaining thought experiment, and Ackerman writes with a gentle, graceful style that befits Martin’s mild character. The tone also suggests something about how we absorb reality. Ackerman’s alternate America accepts its peace dividend almost blithely; if we do ever conquer death, we’ll reduce it to two-minute cable segments picking nits about it. But Martin has to be made almost untenably dim as a narrator to sell the premise. Foote’s notion of a “great compromise” certainly had its adherents, especially after he became an unlikely household name as an avuncular talking head in Ken Burns’s “The Civil War.” But Martin, in his personal life and his research, needs to amass a mountain of evidence before he even contemplates the idea that the Confederacy was rooted in slavery, that “compromise” and “appeasement” are two different things, and that Faulkner, whom he loves so much, really meant that business about the past not being past.

“I’ve never harbored any desire to participate in historical events,” he muses. “I prefer my history in the past as opposed to the present.” Ackerman is writing into current debates about how history is written, exemplified by the New York Times’s 1619 Project and responses such as the Donald Trump administration’s 1776 Commission. But Martin lives in a time when the debate isn’t quite so overt: He’s a symbol of how readily even the well-intentioned can be to keep history trapped in amber.

Beyond that, Ackerman delivers a potent critique of the what-if nature of talking about history in general. Without giving too much of the story away, “Halcyon” returns to some baseline truths. Humanity is only so capable of cheating death. The occupant of the Oval Office only matters to the extent that broader flaws in American life get addressed as well. “Was this about a monument on a battlefield, or was this about one generation ceding its power to the next?” Martin wonders. As if they were two separate things.