Jackson's Valley Campaign
Hardenbergh joined the Union army on August 15, 1861. He was tasked with manning the requa, a primitive type of rapid fire artillery. In 1862, he fought against soldiers under the command of Confederate General "Stonewall" Jackson during Jackson's Valley Campaign. In 1863, he participated in the Siege of Charleston. In January 1864, Hardenbergh's enlistment ended, although he reenlisted following a brief furlough home.