Complex sentence: Home Guard units were a last defense against any invading Union forces, used at times to gather information about invading Union forces troop movements, and to identify and control any local civilians who were considered sympathetic to the Union cause.
Split into simpler sentences: Home Guard units were, essentially, to be a last defense against any invading Union forces. They also were used at times to gather information about invading Union forces troop movements, as well as to identify and control any local civilians who were considered sympathetic to the Union cause.