Why were so many Unions generals replaced in the US Civil War?

Answer 1

The most high profile Union general replacements involved the position of general in chief and generals commanding the Army of the Potomac. The powerful Army of the Potomac faced serious defeats in Virginia causing President Lincoln to find new generals for this army frequently. In the case of generals in chief, four generals held that position in the US Civil War. They were Winfield Scott, George B. McClellan, Henry W. Halleck and lastly US Grant. It is important to remember that these changes in command took place in a war of only four years.