Complex sentence: The army established the post to protect steamboat traffic en route to Fort Benton, which carried passengers and freight to supply swiftly growing boom towns at the site of rich gold strikes in the western mountains of the Montana Territory.
Split into simpler sentences: The army established the post to protect steamboat traffic en route to Fort Benton. The boats carried passengers and freight to supply swiftly growing boom towns at the site of rich gold strikes in the western mountains of the Montana Territory.