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.”...