Columbus doesn't have much to do with the modern Thanksgiving holiday in the United States, other than opening the door for later Europeans to colonize the Americas. There are several contenders for the "first Thanksgiving," but there's one that usually gets top billing. I'll discuss some of its rivals in a moment.

Everyone tends to focus on the 1621 Thanksgiving in Plymouth. This is the famous Pilgrims-and-Indians Thanksgiving, sitting down to a meal together and its generally depicted as a celebration of friendly relations between the colonists and the Wampanoag. The truth of the matter is a bit more complicated than that. As I mentioned in this post last year, the Pilgrims didn't invite the Wampanoag to their festivities. They were celebrating their first harvest and a very successful hunt for waterfowl. During their festivities, some of the colonists engaged in celebratory gunfire. The noise of this gunfire attracted the attention of their Wampanoag neighbors. The colonists and the Wampanoag had previously established a defensive alliance, so - fearing that that either the colonists were under attack or had broken their alliance and were currently launching their own assault - Massasoit Ousamequin, the Wampanoag leader, rallied 90 armed men (substantially more than the people currently living in Plymouth) to investigate the disturbance. Once they realized the colonists were just having a feast, the massasoit instructed his men to kill five deer and presented them as a gift; after which they joined in the festivities.

An earlier Thanksgiving took place on December 4, 1619 at Berkeley Hundred (now Berkeley Plantation) in Virginia. This is one is of the "thankful for a successful trip to America" variety. The colonists of Berkeley Hundred set aside the date of their landing in Virginia as a yearly observance. The original settlement of Berkeley Hundred was effectively dissolved during the Second Anglo-Powhatan War in 1622, as some of the colonists were killed in the fighting and others retreated to more secure settlements like Jamestown. Due to this discontinuity in the settlements history, the Berkeley Thanksgiving has largely been regulated to a historical footnote.

A later, and much grimmer, Thanksgiving tradition began on October 12, 1637, when the New England colonists introduced an official "day of public Thanksgiving." What makes this Thanksgiving particularly problematic is that among the things the colonial authorities listed as being thankful for was the "subduing of the Pequots." Part of the "subduing" during the Pequot War include the most infamous massacre in the early English colonies - the Mystic Massacre on May 26, 1637. The massacre has been immortalized in an iconic engraving from the era, depicting the New England colonists and their Native allies (mainly Narragansett and Mohegan) surrounding a Pequot village (sometimes called Fort Pequot or the Mystic Village). The village was set on fire, and anyone who tried to flee was shot on site. 400 - 700 Pequots were killed in the attack, while the bulk of the Pequot warriors were away.

The Mystic Massacre was followed up with the Fairfield Swamp Fight, when the colonists discovered the location of the Pequot warriors headquarters in a formerly abandoned village in the middle of swamp (not Fairfield, Connecticut). Though their leader Sassacus and 40 others managed to escape, the majority of the warriors there were killed. Sassacus and his remaining followers fled to the Iroquois. The Iroquois soon presented Sassacus' head to the colonists as a gesture of their good will toward them.

In the wake of the Mystic Massacre, Fairfield Swamp Fight and the death of Sassacus the remaining Pequots were scattered and leaderless. While the Pequot War was still technically ongoing when the colonists declared their day of public Thanksgiving, the Pequots could no longer offer any effective fighting force. They sued for peace, and the Treaty of Hartford was signed a year later (September 1638), which called for the complete dissolution of the Pequots. Their remaining numbers would be divided among the victors - the colonists, the Mohegans, and the Narragansetts - and forbidden to call themselves Pequots any longer. Those who were given over the colonists frequent found themselves enslaved and shipped off to work on plantations in Bermuda or the Caribbean. Given the history of the Pequot War and its aftermath, its easy to see how a day of Thanksgiving in commemoration of it can be seen as a celebration of genocide (though, fortunately, it was an incomplete genocide and some Pequots were able to hold onto their identity through their captivity and eventually regain their independence from the victors of the Pequot War). Though the modern Thanksgiving narrative is mostly taken from the 1621 Thanksgiving, the 1637 Thanksgiving did contribute to formalizing the holiday for New Englanders. The traditions of both got blended together into northern holiday that was again reformulated into a national holiday, with an emphasis on supposed enemies working and celebrating together, during the Civil War, which /u/FatherAzerun discussed a bit this post.