This statement in the Declaration of Independence conflicts with the practice of slavery:
"We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness."
If those same rights were given equally to all men, then it would go against God-given rights to hold people in slavery.
We might want to note that in Thomas Jefferson's original draft of the Declaration of Independence, he also had included a strong statement that condemned the evil of the slave trade. Yes, Jefferson himself owned slaves he had inherited, but saw an eventual emancipation of slaves as something that would need to be done over time. The paragraph in the draft of the Declaration said that the King of England "has waged cruel war against human nature itself, violating its most sacred rights of life and liberty" by capturing, transporting and selling human beings from the distant land of Africa. He called the "market where men should be bought and sold" an "execrable commerce" carried on by authority of the British crown. ("Execrable" is an adjective related to excrement -- something extremely nasty.) Georgia and South Carolina would not join in voting for independence from Britain unless the paragraph about the evil of the slave trade was omitted, and so it was omitted from the final version. But the idea that all are created equal and all persons have rights would mean there's no real room for slavery in a country based on rights and freedom.
President Johnson proved to be an obstacle to the Radical Republicans in Congress, who attempted to completely overhaul the Southern government and economy.
