The USA never condoned slavery.
Let's do a walk through history, prior to 1781, we were a British colony and so subject to British laws. After that we were 13 different countries loosely organized as a confederation much like the EU is today. During that time many "nations" (states had already outlawed slavery).
When these independent states gathered for a second time and tried to form a more perfect "union"... in other words, a better EU/NATO organization. So each "country" (state) agreed to grant the Federal government specific limited powers, nothing more. Read Article X very closely, three times if necessary.
And so the United States (the American EU had countries (states) with slavery and without slavery.
Slavery is inherently a terrible economic model and nothing proved that more than the post Civil War boom in agriculture when "freed" slaves worked as sharecroppers rather than slaves. Though even sharecropping was less efficient than the owner operator farms. Simply said, workers work best when working for themselves and second best when working for wages and least when working as slaves.
When the South seceded from the Union, the rest of the states (nations) had the necessary votes to change their more "perfect" union and so passed the 14th Amendment. Again don't make the mistake of viewing the United States as a single country prior to 1865 (it wasn't) and people back then did not see it that way. That is why through our first 100 years, states were constantly talking about leaving... the Hartford Convention being just one example.
Then Lincoln once and for all ended the idea that states were "independent". And from that time forward, the US (now with the "powers" of a country) abolished slavery. Something every American today should take pride in.
Sadly, when the NYT hired an African American history professor to fact check their narrative, they rejected her comments when she did not support the fake narrative, that the NYT was selling.
https://reason.com/2020/03/06/1619-project-fact-checker-nikole-hannah-jones-leslie-harris/#:~:text=%28Screenshot%20via%20CBS%20%2F%20Youtube%29%20Leslie%20Harris%20is,chattel%20slavery%20as%20a%20foundational%20aspect%20of%20America.
The biggest problem with the 1619 Project is the intermixing of fiction with truth. But that is common, when the writers start with the answer rather than let the history take you where it leads.
Perhaps if a group of reputable African American historians went through the work and expunged the opinions and focused on the facts a great deal of benefit could be gained and a far better understanding of the role slavery played in colonial America and later in the South. This knowledge could then be grafted into mainstream American history rather than remain an alternative view of American history that is easily criticized for its gross misrepresentations.
However, if you prefer alternative histories, one of my favorite is how Germans built America. As the single largest immigrant group to come to America during its first 100 years, arguably they were the most important in shaping it especially in the North and Midwest (the South was way to hot for Germans). Of course, I suspect my German heritage makes me inclined to support those views... I mean would America even have existed if Baron von Steuben had not reformed the American military. The truth is America is the product of every ethnic group from every race whose presence made America what it is.
