On May 23, 1863, Wass was appointed lieutenant colonel of his former regiment, the 19th Massachusetts Infantry. He was severely wounded during Pickett's Charge on July 3, 1863 at the Battle of Gettysburg. On October 14, 1863, was given temporary command of the 3rd brigade, 2nd division of the II Corps which he led during the Battle of Bristoe Station. He was wounded a second time during this battle. He was promoted to colonel and commanding officer of the 19th Massachusetts on February 28, 1864, a post which he held until the regiment was mustered out on July 28, 1864. On August 6, 1864, Col. Wass was appointed commanding officer of a new regiment, the 60th Massachusetts Infantry. The regiment served first in Maryland to guard railroad lines and later was transferred to Indianapolis to deal with the threat of organized Confederate sympathizers in that region. After the close of the 60th Massachusetts Infantry's term of service, Wass was appointed colonel of the 62nd Massachusetts Infantry on March 2, 1865 despite poor health due to his long service and wounds. The war ended before the 62nd Massachusetts could take the field.