GETTYSBURG, Pa. — The sun rose on the second day of the Battle of Gettysburg, 2018, to reveal a line of cars parked behind the Union Army’s tents.

It was an annoying historical anachronism for the brigade’s commander, Ted Brennan, 49, who was brushing his teeth with a horsehair toothbrush.

“We try to be as authentic as we can without getting dysentery,” Mr. Brennan said of his unit, several of whom were frying bacon and brewing coffee over a fire. They were camped in a sea of canvas tents that housed many of the 6,000 re-enactors at the event. Beyond the spectator stands and hot dog stalls, the Confederates were camped just out of sight.

The 155th Gettysburg anniversary re-enactment, which was held over the second weekend in July, was a chance for dedicated hobbyists to blast away at each other with antique rifles and rekindle old friendships over campfire-cooked meals. Spectators paid $40 to watch nearly a dozen mock skirmishes over the course of four days, and there was an old-timey ball on Saturday night. An Abraham Lincoln impersonator was on hand to pose for photos.