In 1861, when Lee chose to resign from the U.S. Army, abrogating his officer’s oath and accepting a commission in the Virginia militia, many condemned his action. In fact, when Lee gathered his Unionist family to tell them of his decision, he reportedly acknowledged they would disagree with him.
Other Virginians also questioned his decision. There were eight colonels in the U.S. Army from Virginia at the time the state seceded. All West Pointers, seven remained loyal. Lee and only Lee chose treason, chose to try to destroy the United States. And in doing so, he chose to fight for a new country dedicated to human enslavement. He certainly understood slavery, having spent more than two years from late 1857 to early 1860 running the plantation at Arlington, with its around 200 enslaved workers.
Lee fought for slavery because he believed in slavery.