In the fall of 1958, James McPherson came to Baltimore as a 22-year-old graduate student in American history at Johns Hopkins University. He remembers the pleasant summer nights and walks to Memorial Stadium to watch the Orioles play. There is something else he recalls -- the feeling that, all around him, the world was changing.

"When I arrived here, the restaurants were still segregated, and the schools had just been desegregated," he said. "And, nationally, there were sit-ins, confrontations between federal authorities and local jurisdictions. Federal troops were being brought in to Alabama and elsewhere to enforce the law."

It was heady stuff for James McPherson, who was born in North Dakota and grew up in the small Minnesota town of St. Peter. So, like many a history student before him and since, he saw that one way to understand the present was to examine the past -- "I realized that a lot of this had happened 100 years before." He plunged into study of the Civil War and the Reconstruction period.

That era has retained its fascination for him ever since. Dr. McPherson is concluding his third decade of teaching American history at Princeton University and has become one of the pre-eminent living Civil War historians. His one-volume history of the war, "Battle Cry of Freedom: The Civil War Era," not only was a best-seller for months but won one of two Pulitzer Prizes awarded for history in 1989 (Baltimorean Taylor Branch won the other for "Parting the Waters").

Now he's deep into research on his ninth book, which will focus on the soldiers who fought the Civil War. He will draw on that material tomorrow night in a lecture, "Why They Fought: Ideology and Combat Motivation During the Civil War," that will be delivered at the Maryland Historical Society.

Dr. McPherson had written extensively on the Civil War for many years, but he admits he was not prepared for the reception given to "Battle Cry of Freedom" -- "I was hoping for some good reviews and modest commercial success." A reviewer noted in the Los Angeles Times: "Bright with details and fresh quotations, it must surely be, of the 50,000 books written on the Civil War, the finest compression of that national paroxysm ever fitted between two covers." The book's popular success helped pave the way for the overwhelming reception accorded the 1990 PBS special "The Civil War," for which Dr. McPherson served as a sometime consultant.

With so much having been written about the Civil War, he acknowledges, it's difficult to come up with something fresh. "There's very little new information that's available," Dr. McPherson, 56, said in an interview yesterday. "What you get is mostly new interpretations of what has been written about before.

"For instance, I'm reading now a biography of [Union Gen.] Ambrose Burnside, who generally has been treated rather harshly. This book portrays him in a much more favorable light."

For his new book, which he hopes to complete "in the mid-'90s," Dr. McPherson is relying heavily upon letters written by Union and Confederate soldiers. He came to Baltimore a few days early this week to do research at the Historical Society. (It has a "vast collection" of soldiers' letters and other personal papers, says TC Mary Ellen Hayward, special assistant to the director of the society.)

"Since Maryland was a Border State, the Historical Society has letters from soldiers on both sides," said Dr. McPherson, a youthful-looking man who, not surprisingly, has the air of one used to speaking on his feet -- he summarizes rapidly and the ideas fall together as if in neatly tied bundles. "There's a lot to be found in these letters. I'm finding that ideology had a great deal of importance -- that both Union and Confederate soldiers really believed in what they were doing. Remember that soldiers weren't being drafted until halfway through the war."

Unlike some Civil War historians, such as Shelby Foote, Dr. McPherson did not grow up immersed in the culture and history of the conflict. "I guess I regarded the South as an exotic, puzzling place," he said with a smile. "But I had this general notion I wanted to learn about the South, so that's one reason I chose Hopkins."

Specifically, he wanted to work with C. Vann Woodward, then a professor at Hopkins and already a nationally respected historian, especially for his work in Southern history ("The Strange Career of Jim Crow"). Dr. Woodward left Hopkins in 1961 to teach at Yale and Dr. McPherson went to Princeton the following year, but the two have remained friends.

"He came out of the West from a college I didn't know [Gustavus Adolphus], but very immediately impressed me with his seriousness in his work and his ability," says Dr. Woodward, who retired from Yale in 1977.

"I expected good things from him and I haven't been disappointed.

"Surely his ability accounts for much of his success. I would think he'd admit his subject would account for some of it. Still, many people write about the Civil War, but not many write about it well."

Dr. McPherson's ability to take fresh approaches in an overworked area was apparent in his most recent book, "Abraham Lincoln and the Second American Revolution." Published last year, this collection of essays and lectures puts forth some of his oft-argued beliefs -- that the Civil War was such a profoundly important part of our history that it truly was a "Second American Revolution." It also proposes such theses as the contention that Lincoln was such an effective public communicator because he used metaphors so well: "Many of them are extraordinarily well chosen and apt; they have the persuasive power of concreteness and clarity."

His estimation of Lincoln, Dr. McPherson said, has grown over the decades. "My doctoral dissertation was on the Abolitionists after the Emancipation Proclamation, and I generally was very sympathetic toward them," he said. "They, of course, were very critical of Lincoln -- they thought he moved too slowly. But the more I studied Lincoln, the more I saw that they were just one of several pressure groups, and that he couldn't move as quickly as some wanted because there was the danger of major backlash."

He does concede that occasionally he considers working another aspect of American history -- the American Revolution interests him, as does the Franklin Roosevelt era. And, he said, half-seriously, "Sometimes I look enviously at my colleagues in medieval history. There's such a narrow range of sources to be studied, and they're able to come up with all kinds of creative speculation in their work. I have such a huge number of sources, and so much has been written already."

But he's not ready to look for another line of work yet. "The Civil War era is so fascinating, and the characters are so colorful," he said. "I'll be doing a lot more work on it, I'm sure."

James McPherson will speak on "Why They Fought: Ideology and Combat Motivation During the Civil War," at 8 p.m. tomorrow at the Maryland Historical Society, 201 W. Monument St. The talk is open with admission to the museum: $2.50 for adults, $1.50 for seniors.