Complex sentence: As the British concentrated on the southern United States in 1779, General George Washington finally had an opportunity to do something about the Iroquois actions; he instructed General John Sullivan and about 5,000 men to attack and destroy Iroquois villages in upper New York.
Split into simpler sentences: As the British concentrated on the southern United States in 1779, General George Washington took action against the Iroquois. He instructed General John Sullivan to attack and destroy Iroquois villages in upper New York.