The Man Behind the Gun (1953)

Posing as a schoolteacher, undercover government agent Ransome Callicut arrives in 1850's California to gather intelligence about an insurrectionist plot to have the southern part of the state secede to become a slave state. When he discovers a hidden cache of weapons, he reveals his true identity and assumes command of the local army post. Aided by sidekicks Monk Walker and Olaf Swenson he battles political assassination and other intrigues to unmask the ringleader of the plot and keep the Golden State in the Union.