The Unauthorized Regiment (formally the 1st Montana Volunteer Cavalry Regiment) was a group of volunteers led by Colonel Theodore Roosevelt to patrol the United States border with the Dominion of Canada during the Second Mexican War. They fought the Anglo-Canadian army, commanded by General C.G. Gordon, that invaded Montana Territory from Canada. In conjunction with George Armstrong Custer's Army Regulars, Roosevelt's Unauthorized Regiment inflicted a crushing defeat on Gordon's army.

Controversy rose almost immediately as to who won the Battle of the Teton River -- Custer or Roosevelt. The two men became critical of one another for the rest of their careers -- culminating in a near-physical fight in the closing days of the Great War, when Custer was commander of the victorious First Army and Roosevelt the president of the victorious United States.

In the 1930s, a movie about the Unauthorized Regiment was made with Marion Morrison starring as Roosevelt.