You learned all about it in kindergarten: Thanksgiving started about 400 years ago when the Pilgrims Mayflowered the Plymouth Rocks.
At some point in their quest, they crossed paths with the Indians, who were native non-Indian settlers of the Americas, still referred to as Indians, but settled to be called Native Americans. They made feather hats out of construction paper, glued macaroni to paper plates, and performed a children’s play called Pocahontas. More importantly, they celebrated togetherness and companionship with a giant feast, and by giving things to each other, then thanking each other for the things they gave to each other. Thanks, dude.
As with all relationships, time slowly passed and they quickly got sick of each other. The Indians still thought the Pilgrims were pretty cool, but the Pilgrims thought it would be pretty cool to genocide all of the native Indians. So, they went full-satan and handed out blankets laced with smallpox (don’t click), also known as the Red Plague. Or maybe they didn’t, I don’t know. I wasn’t there. Leave me alone.