The origins of Thanksgiving lie in first contact between the British and Native Americans.
The pilgrims came. The pilgrims starved. The Indians saved the pilgrims from starvation. The pilgrims had a feast to give thanks, at which the Indians supplied the meat, as you can see in this pilgrim's account of the first Thanksgiving.
"Many of the Indians [came] amongst us and... their greatest King, Massasoit, with some ninety men, whom for three days we entertained and feasted; and they went out and killed five deer, which they brought... And although it be not always so plentiful as it was at this time with us, yet BY THE GOODNESS OF GOD WE ARE... FAR FROM WANT."
A few years after that most of the Indians were rounded up and killed, and their land was stolen.
Happy Thanksgiving.
