What was McClellan's lucky break?

McClellan's lucky break came during the Civil War at the Battle of Antietam in September 1862, when he was able to exploit the Confederate Army's miscommunications and delays. His opponent, General Robert E. Lee, had split his forces, which allowed McClellan to confront them separately. Additionally, the discovery of Lee's battle plans, wrapped around a cigar and found by Union soldiers, provided McClellan with crucial intelligence. This advantage ultimately led to a strategic, albeit costly, Union victory and gave McClellan a temporary boost in reputation.