In the summer of 1862, President Lincoln called General Henry W. Halleck to Washington, D.C., to take command of all Union armies in the death struggle against the Confederacy. For the next two turbulent years, Halleck was Lincoln's chief war advisor, the man the President deferred to in all military matters. Yet, despite the fact that he was commanding general far longer than his successor, Ulysses S. Grant, he is remembered only as a failed man,...