Who is General W.H.L. Wallace from Personal Memoirs of U.S. Grant and what is their importance?

A general who was serving during the early part of the Civil War, Grant said that Wallace's arrival during the battle at Pittsburgh Landing cinched a Union victory. Wallace was left with the need to defend Washington, though he had only raw recruits at his disposal. Grant said that Wallace did all that could have been expected of him under the circumstances by delaying the advance of the Confederates by a full day, giving the Union Army time to get in place.