Established in 1856, Fort Simcoe is of outstanding historical significance as one of the few remaining pre-Civil War military outposts in the Pacific Northwest. Built during the height of the Yakima War (1855?1858), the fort was established by the U.S. Army to maintain peace, secure the region for white settlement, and monitor the newly created Yakama Reservation. Under the direction of Major Robert S. Garnett, the outpost was constructed with surprisingly sophisticated Greek Revival-style officers' quarters and sturdy log blockhouses. Designed to project federal authority in the remote interior of Washington Territory, the site is architecturally unique for its preservation of four original wooden officers' quarters, a commander's house, and reconstructed blockhouses, standing as a rare physical testament to the mid-19th-century military frontier.

Following the cessation of hostilities and the departure of military troops in 1859, the property was transferred to the Bureau of Indian Affairs, initiating a long and complex history as the Yakima Indian Agency. For decades, Fort Simcoe served as the administrative headquarters of the reservation and, starting in 1860, as a prominent Indian boarding school. Under the leadership of influential agent James H. Wilbur, the site became a focal point for federal policies aimed at the cultural assimilation of Yakama children. This dual legacy of military occupation and Native American administration makes Fort Simcoe a vital historic site for understanding the collision of cultures, federal Indian policy, and the development of the Pacific Northwest. Today, preserved as a state park, the site honors both its military architecture and its deep significance to the Yakama Nation.