Complex sentence: He witnessed parts of the Battle of the Little Bighorn, and his translated account later appeared in several newspapers, as he was thought to be the only surviving witness from the U.S. side of Custer's Last Stand.
Split into simpler sentences: He witnessed parts of the Battle of the Little Bighorn, and his translated account later appeared in newspaper accounts. He was thought to be the only surviving witness (from the U.S. side) of Custer's Last Stand.