Complex sentence: They began to appoint their own Indian agents, and in some areas built military forts along the northern border, for instance, a fort in 1820 at Sault Ste. Marie, Michigan.
Split into simpler sentences: They began to appoint Indian agents, and in some areas built military forts along the northern border. An example was the fort built in 1820 at Sault Ste. Marie, Michigan.