An article which appeared in Revolutionary Worker #883, November 24, 1996 discusses the truth of the Thanksgiving celebration as having been rooted in massacre of the native peoples of this land. This website is the home of the Revolutionary Communist Party, USA. Therefore one should read this with a grain or two of salt in that their article is going to be slanted to support their cause.
For example the article starts with: Every schoolchild in the U.S. has been taught that the Pilgrims of the Plymouth Colony invited the local Indians to a major harvest feast after surviving their first bitter year in New England. But the real history of Thanksgiving is a story of the murder of indigenous people and the theft of their land by European colonialists--and of the ruthless ways of capitalism.
This article has section headers which reveal the content:
How the Puritans Stole the Land
The Birth of "The American Way of War"
Discovering the Profits of Slavery
Thanksgiving in the Manhattan Colony
The Conquest of New England
Runaways and Rebels