Did Battle, and US Future, Hang on Thread of Fate? From as far away as Minnesota, Colorado and Ohio they came, more than 30 members of the Bloss and Mitchell families who converged on the hallowed Civil War fighting grounds of rural Maryland. John McKnight Bloss, now 81, had just parked his RV at a campground when he tried to sum up what this gathering of his clan was about. He's been researching his namesake great-grandfather, who was wounded four times during Civil War battles, including the epic fight along meandering Antietam Creek 150 years ago - and he wanted the younger generation to "understand the sacrifices that were made." Robert Mitchell Menuet spoke proudly of Barton Mitchell, his ancestor who served alongside John Bloss in the 27th Indiana Volunteer Infantry and suffered a life-shortening wound at Antietam - one of the 23,000 casualties that made the battle on Sept. 17, 1862, the single bloodiest day in U.S. history. But something more particular drew the descendants to Maryland. They cheered the opening last month of an exhibit in nearby Frederick showcasing a simple action their forebears took that helped change the course of the war - and with it, perhaps, the course of America's history as one nation, indivisible. The exhibit's centerpiece was a two-page document - "something that has his fingerprints on it," said Mitchell's great-great-grandson. It's a handwritten copy of Gen. Robert E. Lee's secret Special Orders No. 191, detailing the Southern commander's audacious plans for an invasion of enemy territory that would propel the Confederates to victory. Carelessly left behind as Lee's army marched north, the copy was spotted in a field by the Indianans, and Lee's name jumped out as Mitchell and Bloss read it. When they passed their stunning find up the chain of command, Lee's counterpart, the famously cautious Union Gen. George McClellan, exclaimed, "Now I know what to do!" Four days later came the cataclysmic clash along the Antietam near Sharpsburg - what James McPherson, the eminent Civil War historian, has called "arguably ... THE event of the war. Over years of study, the Princeton professor and Pulitzer prize-winning author has come to rank Antietam and the finding of the lost orders among the most notable moments when America's trajectory turned and its very future was reset. Pondering the "one-in-a-million" opportunity that the Indiana infantrymen seized, McPherson said he understood their family members' excitement. "They can take pride in what they did," he said in an interview, "but also marvel at the accidental nature of it." This is a story about a harrowing battle that let America become the nation it is today, and about the thread of fate on which some say it hung. Americans north and south had endured a year and a half of brutal Civil War. By September 1862, the weariness and worry of its ups and downs showed in soldiers' troubled letters home, in newspapers' jittery overreactions to each development, in the haggard face of Abraham Lincoln.