Complex sentence: They returned back to New York City in July, 1776 after the city had been retaken by the British, not returning to Canada until 1779, a prolonged absence for which he was sharply criticized.
Split into simpler sentences: They returned back to New York City in July, 1776 after the city had been retaken by the British. He did not return to Canada until 1779, a prolonged absence for which he was sharply criticized.