In 1621, Massasoit, sachem of the Wampanoags of New England negotiated a treaty with Pilgrim settlers. A half-century later, as a brutal war flared between the English and a confederation of Indians, this diplomatic gamble seemed to have been a grave miscalculation.

Ahead of the 400th anniversary of the first Thanksgiving, a new look at the Plymouth colony's founding events, told for the first time with Wampanoag people at the heart of the story. In March 1621, when Plymouth's survival was hanging in the balance, the Wampanoag sachem (or chief), Ousamequin (Massasoit), and Plymouth's governor, John Carver, declared their people's friendship for each other and a commitment to mutual defense. Later that autumn,...