This answer is adapted from an earlier answer to a similar question...

In 1620 one hundred and two passengers, many of them separatists from the Church of England, departed their homeland. They arrived at Cape Cod in November, and immediately began looting corn from Nauset communities along the coast before arriving at Patuxet, a recently abandoned village allied with the Wampanoag Confederacy. A small-scale epidemic constricted the population away from Patuxet, and prompted Massasoit, sachem (paramount chief) of the Wampanoag Confederacy, to abandon the long-standing policy of opposing long-term European settlements in his homeland.

The epidemic that struck Patuxet weakened the Wampanoag Confederacy, while leaving their Narragansett enemies unscathed. A map of the southern New England coast provides insight into the challenging political world these religious separatists entered into. Massasoit, hoping to change the shifting power dynamics of southern New England back into his favor, waited the winter before approaching the Plymouth encampment. Samoset, an Eastern Abenaki sagamore (subordinate chief) who learned English from fishermen visiting the Gulf of Maine, journeyed south to Plymouth and made “first contact” with the strangers. He returned a few days later with Massasoit and Tisquantum/Squanto. Tisquantum, a Patuxet, was kidnapped by Englishmen in 1605 and again shortly after his return to Massachusetts in 1614. During his odyssey to return home Tisquantum crossed the Atlantic six times, and finally returned to Massachusetts in 1619.

For the colonists the situation was dire. Roughly half of The Mayflower passengers perished from hunger and disease during the first winter. To the starving, frightened inhabitants of Plymouth the arrival of Massasoit, Tisquantum, and Samoset proved a godsend. In a pattern reminiscent of first contacts throughout the Americas starving colonists depended on the goodwill of indigenous communities for permission to settle, expert political and geographic knowledge to navigate through a New World, and food trade to survive. Tisquantum functioned as interpreter and intermediary, teaching and guiding the new arrivals. Governor William Bradford called him “a spetiall intruments sent of God.” With Tisquantum and Samoset’s assistance, Massasoit and Bradford developed a peace treaty. Neither party would do harm to the other. If one was attacked by an outside party, the other would come to their aid, and if a Wampanoag broke the peace he would be sent to Plymouth for punishment, as a colonist would be sent to the Wampanoag if he violated the peace. By the end of the harvest the

governor sent four men on fowling, that so we might after a special manner rejoice together after we had gathered the fruits of our labor. They four in one day killed as much fowl as, with a little help beside, served the company almost a week. At which time, amongst other recreations, we exercised our arms, many of the Indians coming amongst us, and among the rest their greatest king Massasoit, with some ninety men, whom for three days we entertained and feasted, and they went out and killed five deer, which we brought to the plantation and bestowed on our governor, and upon the captain and others. (Edward Winslow)

That us our documented history of the first Thanksgiving. In Massachusetts the promise of peace and Thanksgiving lasted less than a generation. The arrival of more land-hungry colonists, the constant assault on indigenous territory, and the transformation of the New England ecology created a toxic colonial world. The Pequot War established the English precedent of total war against Native American rivals. Survivors, combatants and non-combatants alike, could expect punishment and enslavement if they dared oppose English demands. Massasoit, sachem of the Wampanoag, hoped forging alliance with Plymouth would secure their access to valuable trade partners. Massasoit’s son, Metacomet/Phillip, would die in a war that nearly threatened the survival of English interests in Massachusetts. After his death Metacomet/Phillip's head was mounted on a pike at the entrance of Fort Plymouth where it stood for two decades. His wife and children were enslaved and sold to the West Indes.

Sources

Bragdon Native Peoples of Southern New England, 1500-1650

Calloway First Peoples: A Documentary Survey of American Indian History

Cameron, Kelton, and Sedlund, eds. Beyond Germs: Native Depopulation in North America

Mann 1491: New Revelations of the Americas before Columbus

Newell Brethren by Nature: New England Indians, Colonists, and the Origins of American Slavery

Richter Before the Revolution: America’s Ancient Pasts