Built in 1822 in Sabine Parish, Fort Jesup was created to guard the United States border with New Spain and bring order to the lawless Neutral Strip left by an unresolved boundary after the Louisiana Purchase. Established by Colonel Zachary Taylor after the Adams–Onís Treaty fixed the border at the Sabine River, the post served as the base for his Army of Observation, which subdued the former Neutral Strip and monitored events in Texas. In 1845 Taylor marched from here into Texas, helping set in motion the Mexican–American War. Closed in 1846, the site was later restored and became a National Historic Landmark.