Sit down for a while with any Civil War buff and sooner or later you’ll probably end up debating the same question.

What if Gen. Robert E. Lee had won the battle of Gettysburg?

In the annals of American history it’s one of the great “what ifs.” A subject that has been hashed and rehashed a million and one times by those who simply can’t help but let their mind consider the implications of changing the outcome of that one battle, which was so narrowly decided on July 3, 1863.

If the Army of the Potomac had been defeated at Gettysburg, could Lee have marched on Washington, D.C.? And if so, would President Abraham Lincoln have been forced to seek peace with the Confederacy? And if he had, could America have existed as two separate nations, or would it continue to split and divide itself again and again?

This sort of alternative history speculation is in some ways the guilty pleasure of historians — it’s not the standard names-and-dates way to study the past but a chance to let one’s mind run wild with that wonderfully open-ended question, “What if?”

In their previous novel, “Gettysburg,” former Speaker Newt Gingrich (R-Ga.) and William Forstchen, an associate professor of history at Montreat College in North Carolina, decided to finally write a plausible scenario for Lee’s defeating the Army of the Potomac. Now, in their latest novel,“Grant Comes East,” which was published last month by St. Martin’s Press, Gingrich and Forstchen return to give a feasible next step in their version of the Civil War. This “alternative history” takes the reader from the political intrigues of Tammany Hall right to the chaos of a predawn charge in creating battles and events that almost were.

“We call this active history to distinguish it from normal history,” Gingrich said in a recent interview. “It teaches you to think actively about events, not passively. You’ve really got to know your history to do it.”

And what these two storytellers have created is a fascinating tale derived from Gingrich’s political savvy (and a military mind developed while working as the longest-serving teacher of the “Joint War Fighting” course for major generals), Forstchen’s background in studying the life of the common solder, and the knowledge of a half-dozen other Civil War afficionados who came together for this project.

But if telling an alternative history is an exercise in creativity, Gingrich and Forstchen have proved that it is also a heck of a research project.

In order to create as believable a story as possible, the two authors not only had to get into the minds of their major characters — from Lee to Lincoln to Jefferson Davis, Ulysses S. Grant and Dan Sickles — they also had to figure out the goals, personalities, strengths and weaknesses of each man. (Would Lincoln ever have considered abandoning Washington? How much would Sickles’ desire for political office influence his battlefield decisions?) Then Gingrich and Forstchen had to create a credible strategy that took into account accessibility of travel, terrain and weather conditions from their specific time frame as well as consider the innate limitations of a mid-19th-century army.

“We had this huge map from the period that we blew up and with stacks of reference books all over the floor we crawled around and hashed out a strategy,” Forstchen recalled.

In the course of their debates, members of the group — who Forstchen admits would often have their wives shaking their heads when they found their husbands crawling around on the floor discussing the order of battle of an engagement that never happened — became so caught up in their fictional summer of 1863 that they would find themselves calling and e-mailing each other into the wee hours of the night.

“I think we meet the time and space problems pretty darn accurately,” Gingrich said. “We’ve created something that we think will stand up under pretty intense scrutiny.”

“It’s a fascinating process because we’re all historians and we all have different perspectives,” Forstchen said. “There really is an ‘Aha!’ moment when we would all sit back and grin and say, ‘That’s it.’”

With Lee fresh from a victory outside Gettysburg and the Army of the Potomac reeling in defeat and fleeing across the Susquehanna River, “Grant Comes East” begins with the Confederate Army marching southward, now with a six- to eight-week window of opportunity to operate without any major Union army in the field to oppose it.

While Grant and his army in the West rush eastward to save the war, Lee decides he must try to take Washington, D.C., knowing this is his one chance to end the war in one swift coupe de main. And at the same time that Lee carries out his assault on the most fortified city in America, others in the Confederate government are looking for ways to bring Maryland into the Confederacy and unleash the Southern sympathy that had been on the verge of boiling over in the Old Line State since the start of the war.

In the course of “Grant Comes East,” the authors tackle a number of key military and social issues from that time, including foreign intervention from France and England, anti-war and draft rioting in Northern cities, and questions about the use of black soldiers on both sides of the battlefield. Parts of the story are told from the perspective of John Miller, a black man born free in Baltimore who joins the Union army, and even France’s Emperor Napoleon III, who makes an appearance as he decides whether to join in the fight in America.

Maps of Lee’s movement through Maryland allow the reader to envision the campaign as it unfolds from the days following Gettysburg through the rest of that summer’s campaign. Also sprinkled throughout the book are photographs — mostly from the Library of Congress’ collection — of Union defense works, marching soldiers, broken bodies littering battlefields and portrait shots that add vivid real images to this fictional story.

One interesting tidbit is that Forstchen and Gingrich’s wife appear in the book, both posing for portrait photographs in period costumes. The two pictures were taken at an authentic Civil War photographic studio located in Gettysburg with a camera that dates back to the 1860s.

After a brief book tour for “Grant Comes East,” including a recent stop in Gettysburg, Gingrich and Forstchen will soon roll out the maps, reassemble their advisers (which include local historians, a historical artist and a former commandant of the Army War College) and figure out the ending to the three-part series. With a working title of “War to be Won,” the next book is due out around Father’s Day next year.

“For a day and a half we’re going to fight out book three,” Gingrich said, noting that he’s looking forward to finishing the war he and Forstchen began with “Gettysburg.”

“I just think it’s fun,” Gingrich added. “It forces you to stretch your mind. … We both love history. We love teaching it and we love studying it. It drives us crazy when it’s just memorization.”