He served on many garrison and fortification details, most notably participating in the construction for defenses in New York City, New Orleans, and Pensacola. During the Mexican War, he led the construction of United States Defenses at Tampico. With the outbreak of the Civil War, General Barnard served as Chief Engineer to General Mcdowell in the Firt Bull Run Campaign. Next, with the rank of Brigadier-General, he acted as the Chief Engineer to the Army of the Potomac in the Virginia peninsular, serving Major General George B. McCellan. When the confederate army advanced into Virginia, he was appointed Chief Engineer of the defenses of Washington, and was promoted to Lieutenant-Colonel of Engineers in March 1863. Fanny says he was exceedingly deaf (for obvious reasons) and could not aptly converse with him. FAS Diary Monday, January 12, 1863