was transferred to the Army of Virginia, under Maj. Gen. John Pope, for the Northern Virginia Campaign. In the Battle of Cedar Mountain, Banks's Corps was again up against Jackson, and was again defeated. The corps was at Bristoe Station and did not participate in the Second Battle of Bull Run. Williams's division rejoined the Army of the Potomac as the 1st Division of the XII Corps and marched north in the Maryland Campaign to the Battle of Antietam. On the way, troops from the division found the famous Confederate "lost dispatch," Special Order 191, that revealed Gen. Robert E. Lee's plan for the campaign and gave Maj. Gen. George B. McClellan key insights on how to defeat Lee's divided army. The division was heavily engaged at Sharpsburg, once again up against Jackson on the Confederate left flank. The corps commander, Maj. Gen. Joseph K. Mansfield, was killed early at Antietam, and Williams assumed temporary command. The corps suffered 25% casualties in assaulting Jackson, and Brig. Gen. George S. Greene's division was forced to withdraw from its advanced position at the Dunker Church. George McClellan assigned Maj. Gen Henry Slocum permanent command of the XII Corps after the battle. Williams's division missed the next major battle for the Army of the Potomac, the Battle of Fredericksburg, because it was engaged in defending the Potomac River in the Reserve. In the Battle of Chancellorsville, on May 2, 1863, Stonewall Jackson's corps executed a surprise flanking movement and smashed into the right flank of the Army of the Potomac, severely damaging the unsuspecting XI Corps. The neighboring division, under Williams, entrenched hastily and was able to stop the Confederate advance before it overran the entire army, but it suffered 1,500 casualties in the process. In the Battle of Gettysburg, Williams's division arrived on the battlefield late in the afternoon of July 1, 1863, and occupied Benner's Hill, east of the town of Gettysburg. On July 2, the XII Corps took up positions on Culp's Hill, the right flank of the Union line. At this point, due to a command misunderstanding, Henry Slocum believed that he was in command of the "Right Wing" of the army, consisting of the XI and XII Corps. Therefore, Williams assumed temporary command of the XII Corps and controlled it for the rest of the battle. Brig. Gen. Thomas Ruger took command of Williams' division. On the afternoon