Final answer:
General George B. McClellan was known for his cautious approach to battle, which led to missed opportunities and frustration on the part of Lincoln, who later replaced him. General Ulysses S. Grant, in contrast, was aggressive and decisive, leading to significant Union victories. The Battle of Antietam highlighted McClellan's reluctance to take risks, resulting in his dismissal.
Step-by-step explanation:
General George B. McClellan was the general in chief of the Union army during the early part of the American Civil War, known for his caution in engaging the Confederate forces. He was in charge of the Army of the Potomac, believing incorrectly that Confederate forces were too superior, thus avoiding risks to his troops. His popularity with the men was in sharp contrast to the frustration felt by President Lincoln and Congress who demanded more decisive actions on the battlefield.
On the other hand, General Ulysses S. Grant's character vastly differed from McClellan's. Grant was recognized for his aggressive tactics and his willingness to engage with enemy forces, which eventually led to a series of Union victories that decisively altered the course of the war. Unlike McClellan, who was overly cautious and often criticized Lincoln, Grant focused on securing victories and pushing the Union's advantages.
The defining moment for McClellan's career might be the Battle of Antietam on September 17, 1862, which had the highest number of casualties in a single day during the war. McClellan's failure to capitalize on his numerical superiority and his inability to decisively crush Lee's forces led to his dismissal by Lincoln, which underscored their contrasting military strategies and abilities.