Complex sentence: In relatively unsettled parts of the United States there was a considerable amount of a certain kind of brigandage, in early days, when the travel routes to the American West were infested by highwaymen.
Split into simpler sentences: In relatively unsettled parts of the United States there was a considerable amount of a certain kind of brigandage. In early days of the United States the travel routes to the West were infested by highwaymen.