Complex sentence: A group of brethren from north of Marietta, Pennsylvania on the east side of the Susquehanna River came to be known as the "River Brethren," and distanced themselves from Boehm and the United Brethren movement.
Split into simpler sentences: A group of brethren from north of Marietta, Pennsylvania on the east side of the Susquehanna River came to be known as the "River Brethren". They distanced themselves from Boehm and the United Brethren movement.