For something that happened 150 years ago, the Battle of Gettysburg still generates its share of controversy. And myth, according to historian Allen Guelzo, “grows like weed out of controversy.”

Guelzo, a professor of history at - appropriately enough - Gettysburg College, is the author of the recently published “Gettysburg: The Last Invasion.” He spoke with ABC News Political Director Rick Klein about the battle and his book – an exhaustively researched and detailed dive into the pivotal fight of the Civil War.

Among the myths of Gettysburg that Guelzo debunks is that the battle was an accident – that Robert E. Lee’s Army of Northern Virginia and George Gordon Meade’s Army of the Potomac merely happened upon each other in the hills of South Central Pennsylvania. “No, it was not really an accident,” said Guelzo. “At least not more of an accident than any battle in the Civil War was.”

Guelzo’s book also restores the reputation of J.E.B. Stuart, cavalry commander of the Confederate Army. “Did he [Stuart] really render Robert E. Lee blind by riding on a joy ride almost entirely out of the campaign?” Guelzo asked. Over the years, Stuart has come in for much criticism for his cavalry’s supposed abandonment of Lee’s main force. “That also is an exaggeration, if not an outright myth.”

“There are a number of other myths,” Guelzo offered. “That the 20th Maine volunteers and Joshua Lawrence Chamberlain saved the Union at Little Round Top. Well, all honor to Chamberlain and his men, they did do the right thing at Little Round Top on July 2nd,” by defending their position with a bayonet charge.

“But actually,” according to the historian, “Chamberlain was one of only a series of junior officers who make decisions like that…[to] save the future of the army and the country, at Gettysburg. He is one face in a crowd of people who make those spontaneous right decisions.”

Guelzo described the battle as “a galaxy of marvelous stories. It’s about people fighting small-scale fights within the big battle, and they’re doing it very much on their own initiative, they’re doing it without direction from famous-named generals, and…they’re making the right decision on their own by just doing it, time after time after time.”

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Placed next to more recent fighting, like that seen in Iraq or Afghanistan, Guelzo suggested “you won’t recognize what combat in Gettysburg looked like. The Battle of Gettysburg was more a species of the old Napoleonic kind of conflict, the old 18th century kind of war, than it was modern warfare. Simply because the weapons technology of the period dictated that.”

Communications technology was different as well, of course. In an age when the idea of telecommunications was confined to telegraphs and hand-delivered messages, even President Lincoln had no way of knowing the outcome of Gettysburg until 24 hours after the battle was over. Guelzo likened the president to a “prospective father in the waiting room. Trying to find out about this big event that’s occurring that he can’t see, or can’t be part of, that he just has to wait for.”

Lincoln got word of the Union victory at Gettysburg at nearly the same moment he also heard of the surrender of the Confederate citadel at Vicksburg. “Those two victories together give Abraham Lincoln the best weekend that he’s had during the war, if not during his life,” said Guelzo.

Gettysburg was “a decisive moment in the middle of the decisive moment of the American Civil War…Up to that point, the Confederacy have been putting on quite a good show for its bid for independence. These breakaway Southern states had formed their own government, formed their own armies, they had sustained defeats, but in the East, the main Confederate Army under General Robert E. Lee had gone from victory to victory to victory,” said Guelzo.

At Gettysburg, “the opportunity was there for Robert E. Lee to win a major victory over a Union army. And if he did that, then the political fallout for that victory for the Confederates might in fact have forced President Lincoln to the negotiating table.”

ABC News' Arthur Niemynski contributed to this episode.