The Department of New Mexico was a department of the United States Army during the mid-19th century. It was created as the 9th Department, a geographical department, in 1848 following the successful conclusion of the Mexican–American War, and renamed Department of New Mexico in 1853. It had to contend with an invading Confederate force during the New Mexico Campaign of the American Civil War from mid-1861 to early 1862, then with Apache tribes during the remainder of the conflict. It was merged into the Department of California after the end of the war as the District of New Mexico.