Complex sentence: Lt. Col. William Stacy, a high-ranking officer of the Continental Army captured at the attack on Cherry Valley, New York by Butler's Rangers, was held captive at Fort Niagara during the summer of 1779.
Split into simpler sentences: Lt. Col. William Stacy, a high-ranking officer of the Continental Army, was captured at the attack on Cherry Valley, New York by Butler's Rangers. He was held captive at Fort Niagara during the summer of 1779.