Answer:
As a military leader, George McClellan was known for organizing the Army of the Potomac during the Civil War.
Explanation: After graduating, McClellan was appointed second lieutenant in the engineering corps. In the war against Mexico, he earned the insignia of first lieutenant and then captain for his dedication, courage and ability to build bridges and roads on the army's march routes. He was also a West Point instructor for a period of 3 years.
Other achievements of McClellan include the lifting of possible routes of the first US transcontinental railroad. As a member of the officer corps, he was sent out of the country to study the European armies and as an observer in the Crimean War. This last one derived in the design of the "chair of McClellan" that was standard equipment in the cavalry of the time until the advent of the mechanization eliminated horses of the service in 1942.
In 1857, he resigned his commission as captain of the First Cavalry to become the chief engineer of the Central Railroad of Illinois, where he occasionally worked with a lawyer named Abraham Lincoln.
When the Civil War began, he resided in Ohio, where he served as president of the Ohio and Mississippi Railroad. McClellan had proven himself as an efficient organizer with a strong personal magnetism. For this reason, together with some successes in West Virginia, President Lincoln appointed him Major General in the Union Army, only under General Winfield Scott. He reorganized a disunited and poorly disciplined army, which earned him the respect and approval of his men. Their military operations, however, were only a series of failures and wasted opportunities. McClellan continually overestimated the capacity of his adversary and his plans, excessively cautious, lacked of the necessary audacity and decision.
After the Battle of Antietam, he was dismissed and ordered to hand over command to General Ambrose Burnside and return to New Jersey, his home at that time, to await new orders, which never came.