@Cycloptichorn,
North America and South America are the names of continents.
No inhabitants of North or South America were American citizens of the United States of America before March 4, 1789.
American citizens did not exist prior to March 4, 1789, because the United States of America did not exist prior to March 4, 1789.
Slavery did not exist in the United States of America prior to its existence, but did exist in North America.
Slavery was not legal in the United States of America until the United States of America existed.
Slavery was legal in the United States of America from March 4, 1789 to December 6, 1865.
Slavery ceased being legal in the United States of America on December 6, 1865.