McClellan was born December 3, 1826 and ranked second in the West Point Class of 1846. He fought in the Mexican War and remained in the military afterward, serving with distinction. He was the official American observer of the Crimean War, and invented the McClellan saddle, which is in use in the US military to this day. He resigned in 1857 and became the president of a railroad company. Because of his high military standing and his practical organizational experience, the governors of Ohio, Pennsylvania, and New York all asked him to command their militia. He accepted the command of the Ohio militia on April 23rd.
As we will see over the next few years, McClellan was a master of organization, but his fighting skills did not measure up. Many times even after winning a battle with superior forces, he would retreat and loose the campaign. Lincoln summarized his by abilities saying, "If he can't fight himself, he excels in making others ready to fight."