John Harris was commissioned a second lieutenant in the Marine Corps on 23 April 1814 and promoted to first lieutenant two months later. He joined the Marines of the USS "Guerriere" under the command of Commodore John Rodgers that summer at Charlestown, Maryland, and served with the forces that opposed the British advance on Baltimore. He was ordered to the defense of Washington but then ordered back to Baltimore before reaching Bladensburg. He wrote his brother "I could see plenty of red coats but could not get within musket shot of them." He described the attack on Fort McHenry as "the handsomest sight I ever saw... to see the bombs and rockets flying and the firing from our three forts."