Peter Hains fought with distinction during the War of Secession, particularly at Hanover in May of 1862. After the USA's defeat at Camp Hill, Hains stayed on in the army.
In 1881, he was now a colonel in the US Cavalary, stationed at Tucson, New Mexico Territory. When the Second Mexican War began later that year, he was the commanding officer of a combined regiment of US Cavalry and Volunteer Cavalry. His military career in the war ended in disaster as he allowed his subordinate commander, brevet Colonel Virgil Earp to convince him to pursue a group of Apaches into an ambush.
In the ensuing battle, his cavalry regiment was decimated and when the combined Apache and Confederate Force pushed home their attack, Hains finally called a retreat. He just barely managed to escape back to Tucson were he remained for the rest of the war.