Over the last couple of decades, I have heard all sorts of claims made about the first Thanksgiving--including the claim that it was a celebration of a massacre of the Indians.
First of all, they didn't call it Thanksgiving. It was a traditional English harvest feast--especially important since so few had survived that first very hard winter of starvation and disease. Some of my ancestors survived that winter, and were part of that feast.
It was not a celebration of victory over the Indians. There is a detailed account in Winslow's December 11, 1621 letter to a friend in England, describing what happened, and the circumstances:
Our harvest being gotten in, our governor sent four men on fowling, that so we might, after a special manner, rejoice together after we had gathered the fruit of our labors. 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, Massasoyt, with some ninety men, whom for three days we entertained and feasted ; and they went out and killed five deer, which they brought to the plantation, and bestowed on our governor, and upon the captain and others. And although it be not always so plentiful as it was at this time with us, yet by the goodness of God we are so far from want, that we often wish you partakers of our plenty.
We have found the Indians very faithful in their covenant of peace with us, very loving, and ready to pleasure us. We often go to them, and they come to us. Some of us have been fifty miles by land in the country with them, the occasions and relations whereof you shall understand by our general and more full declaration of such things as are worth the noting. Yea, it hath pleased God so to possess the Indians with a fear of us and love unto us, that not only the greatest king amongst them, called Massasoyt, but also all the princes and peoples round about us, have either made suit unto us, or been glad of any occasion to make peace with us; so that seven of them at once have sent their messengers to us to that end.