The site for Presidio Santa Cruz de Terrenate was selected on August 22, 1775, by an unlikely architect of Spanish empire: Colonel Hugo O'Conor, born in Ireland, charged with relocating fortifications along the Sonoran frontier. O'Conor chose well in purely tactical terms. The spot commanded a bluff overlooking the San Pedro River, with natural fortifications on three sides. The garrison built walls in the shape of a large square, a triangular bastion at one corner, a two-story guardhouse protecting the main gate. Adobe structures and jacals housed the fifty-six soldados de cuera, literally "leather jacket soldiers," named for the layered deerskin coats that served as primitive armor. But the fort was still incomplete when the Apache came. Artillery had not yet arrived.

The Apaches had no intention of waiting for the Spaniards to finish their fort. They attacked anyone who ventured out for water, burned the crops in the fields, ran off the horse herds whenever they found them unguarded. Modern archaeologists Deni Seymour and Mark Harlan have revised our understanding of what happened on July 7, 1776. The battle did not occur at the fort as long believed. Spanish scouts spotted Apache warriors fording the San Pedro River a day and a half's journey from the presidio. Though unprepared, the soldiers pursued. It was a fatal mistake. Armed with bows and arrows, the Apache resisted, then counterattacked, pushing the Spaniards back. When Tovar's men finally crossed the river, they were overwhelmed. The commander fell with 29 of his soldiers. Only a handful survived to tell the story.

In August, the fort finally received its shipment of weapons. Captain Francisco Ignacio de Trespalacios replaced Tovar and brought reinforcements, swelling the garrison to 83 men. But the reprieve was temporary. In mid-November 1776, Trespalacios led 30 soldiers to the mission of Magdalena in present-day Mexico. They arrived to find that 40 Apache warriors had looted the settlement, murdered the inhabitants, and burned the church, an atrocity that became known as the Magdalena Massacre. The attacks on Fort Terrenate grew relentless. On September 24, 1778, Trespalacios himself was killed along with 27 of his soldiers. Between November 1778 and February 1779, 39 more soldiers died. In May 1779, yet another commander, Captain Luis del Castille, fell.

In just four years of operation, Presidio Santa Cruz de Terrenate consumed 98 Spanish lives, including three commanders. The garrison that was supposed to pacify the frontier became a death sentence for those assigned to it. The Apache proved that European military technology and tactics meant little in their desert homeland, where they knew every canyon, every spring, every path of ambush. The presidio was eventually abandoned, a monument to the limits of colonial power. Today, the ruins lie silent on their bluff above the San Pedro River, marking one of the bloodiest chapters in Arizona's Spanish colonial history, a story that began on the very day America declared its birth.