"Modern Thanksgiving was first officially called for in all states in 1863 by a presidential proclamation of Abraham Lincoln. [...]Lincoln proclaimed a national Thanksgiving on the final Thursday in November, explicitly in celebration of the bounties that had continued to fall on the Union and for the military successes in the war." - https://en.wikipedia.org/wiki/Thanksgiving

It's easy to see why the "Southern States Saw Thanksgiving as an Act of Nothern Aggression" from Lincoln's action. He extended the meaning of the celebration to include victory of the North over the South, and asked the South to observe it, according to wikipedia.