Cavalry soldier and horse at the 2010 Battle at Fort Stevens Civil War Reenactment, Oregon. Fort Stevens was a Union outpost during the Civil War built to protect the entrance to the Columbia River and the cities of Astoria and Portland from possible attack from the British. During the 1860s the NW Territories ownership was still in dispute and the British were suffering due to the North not allowing the South to export cotton. The Union felt there was a real threat of British attack thus building the fort. The fort stayed active up through World War II.