The history of the 45th Brigade Combat Team can be traced back to 1890 with the formation of the Militia of the Territory of Oklahoma. That militia was mobilized in 1898 during the Spanish–American War but never deployed. In 1916 the First Oklahoma Infantry Regiment deployed for border security duty during the Mexican Border Conflict. In 1917, the First Oklahoma Infantry Regiment, reassigned as part of the 142nd Infantry Regiment of the 36th Division fought in the final month of World War I.