After garrisoning Missouri for the fall and winter, the 18th Indiana was attached to Grant’s army endeavoring to cross the Mississippi and seize Vicksburg. The 18th Indiana fought at Port Gibson and Champion Hill and took part in the siege operations around Vicksburg, where he commanded a brigade commended for valorous actions during the initial assault upon the enemy fortress in May 1863. After the successful end of the Vicksburg operation, Washburn’s brigade remained with the XIII Army Corps involved in operations up the Teche River, and the successful capture of several Rebel forts. In January 1864, the 18th Indiana was sent to Virginia where it participated in several stiff skirmishes. In August 1864, the 18th Indiana was attached to the 4th Brigade, 2nd Division (Grover) of Emory’s XIX Army Corps.