At the outbreak of the Civil War, Wilder organized a light artillery company in the Greensburg area, even going so far as to cast two six-pounder cannons at his foundry. Wilder's company was mustered into state service but the Federal government declined to accept it, as Indiana had already met its quota of artillery units. Instead, Wilder was commissioned as a captain, and his men were organized as Company A, 17th Indiana Infantry Regiment at Indianapolis. When the 17th Indiana left for western Virginia in July 1861, Company A took along the two cannons. Once in Virginia, the old Company A was permanently detached and eventually reorganized as the 26th Independent Battery Indiana Light Artillery but was commonly known as the "Wilder Battery" in recognition of its first commander. Meanwhile, Captain Wilder was quickly promoted to lieutenant colonel of the 17th Indiana on 4 June 1861, serving where he remained during the early campaigns in Virginia and garrison duty in Kentucky. In March 1862 Wilder was promoted to colonel, becoming the commander of the 17th Indiana. During the campaign to take Corinth, Mississippi, Wilder quickly earned a reputation as a competent, and even gifted, regimental commander.