Thanksgiving is an American holiday with its origins in the 1620's. Despite the various fables attached to it, it is simply a harvest festival to give thanks for the bounty produced by the hard work of the English colonists.
It was a "thank God we made it" kind of a thing. A feast that marked the end of farm work and the onset of winter. Many cultures have this sort of semi-religious festival.
It is NOT a nationalistic holiday. To most Americans Thanksgiving is a family holiday. Traditional foods, the Macy's T-Day parade, football, a bit of drinking but mostly a time of family, of talking and of closeness.
While the subsequent treatment of Native Americans is tragic and deplorable, the English were dependent on their Indian friends in the 1620's.
An excellent book, "Mayflower: A Story of Courage, Community, and War" by Nathaniel Philbrick gives a fair, and objective portrayal of New England life in the 1620's. You can get a good idea of how cultures clash and religious BS will screw up people. It is available in Kindle
format.