On April 28, 1862, Greene was appointed brigadier general of volunteers and served on the staff of Maj. Gen. Nathaniel Banks in the Shenandoah Valley campaign against Stonewall Jackson. At age 61, Greene was one of the oldest generals in the Union army and his troops took to calling him "Old Man" or "Pap" Greene. (There were actually 17 general officers in the Civil War older than Greene.) However, his age did not keep him from being one of the most aggressive commanders in the army. He commanded the 3rd Brigade, 2nd Division, II Corps, of the Army of Virginia at the Battle of Cedar Mountain during the Northern Virginia Campaign. Attacked by a Confederate force three times the size of his own, Greene and his men refused to give ground, holding out until the neighboring Union units were forced to withdraw. His division commander, Brig. Gen. John W. Geary, received a severe wound during the action and Greene took command of the division temporarily.