While the Confederate leader Jefferson Davis was a West Point graduate, Mexican War hero and former secretary of war, Lincoln had only a brief and undistinguished period of service in the Black Hawk War (1832) to his credit. He surprised many by proving to be a more than capable wartime leader, learning quickly about strategy and tactics in the early years of the Civil War, and about choosing the ablest commanders. General George McClellan, though beloved by his troops, continually frustrated Lincoln with his reluctance to advance, and when McClellan failed to pursue Robert E. Lee’s retreating Confederate Army in the aftermath of the Union victory at Antietam in September 1862, Lincoln removed him from command. During the war, Lincoln drew criticism for suspending some civil liberties, including the right of habeas corpus, but he considered such measures necessary to win the war.