Final answer:
General McClellan squandered the opportunity to decisively defeat Lee's Confederate Army at Sharpsburg (Antietam) due to his cautious strategy, missing the chance to press his numerical advantage which could have shortened the Civil War.
Step-by-step explanation:
The major opportunity squandered by General McClellan after the Battle of Sharpsburg (Antietam) was his failure to capitalize on the numerical superiority of the Union Army over the Confederate force. The Union Army, commanded by McClellan on September 17, 1862, clashed with Lee's Confederate forces in the bloodiest single day of the Civil War.
Despite having a force nearly double the size of the Confederate Army, McClellan's overly cautious strategy led him to hold back significant portions of his army rather than pursue a decisive victory. This cautious approach allowed General Lee's army to retreat into Virginia, extending the war's duration and leading to McClellan's eventual dismissal.
The consequence of McClellan's actions was significant as it allowed the Confederates to recover from what could have been a devastating defeat.
President Lincoln, in need of a clear Union victory, was greatly dissatisfied with the outcome and replaced McClellan with General Ambrose E. Burnside. Despite the high casualty rate and the strategic advantage held by the Union Army, the opportunity for a conclusive victory that could have significantly shortened the war was missed due to General McClellan's cautious approach at Antietam.