An army of visitors a quarter million strong, including legions of Civil War re-enactors, is converging on Gettysburg, Pa., to mark the 150th anniversary of the nation's bloodiest battle, a three-day clash that helped turn the tide of the war.

Areas surrounding the town of 7,000 in southern Pennsylvania are being transformed into battlefield scenes, complete with an outdoor field hospital where hundreds of people acting as surgeons will pretend to triage people acting as wounded soldiers, all while period-dressed guides explain the scene.

"It's our Olympic moment," said Andrea DiMartino, a coordinator with the Gettysburg Anniversary Committee, which stages re-enactments each year. Over four days, the group expects 60,000 to 80,000 spectators, who will pay $40 a day to view the action from stadium seating or from their own blankets or lawn chairs. This year, the group also is offering, for $13, a live broadcast of a re-enactment of Pickett's Charge to be viewed on a computer, tablet or smartphone.

The tourism agency for Adams County, Pa., expects the surge of visitors to inject $100 million into the region's economy....