Final answer:
Union General McClellan found out about General Lee's plans through the discovery of Special Order 191 at an abandoned Confederate camp.
Step-by-step explanation:
During the American Civil War, Union General George B. McClellan found out about General Robert E. Lee's plans through a set of fortunate circumstances. In September 1862, a copy of Lee's detailed orders, known as Special Order 191, was discovered by a Union soldier in an abandoned Confederate camp. The soldier promptly handed the orders over to McClellan, giving him insight into Lee's strategy and enabling him to position his troops accordingly.
Learn more about The significance of the Battle of Antietam in halting the Confederate advance