Complex sentence: The good relationship between the Wampanoag and the Pilgrims lasted, and when Massasoit became gravely ill in the winter of 1623, he was nursed back to health by the English.
Split into simpler sentences: The good relationship between the Wampanoag and the Pilgrims lasted. When Massasoit became gravely ill in the winter of 1623, he was nursed back to health by the English.