The 13th, 14th and 15th amendments to the United Stats constitution
are collective known as the "slave amendments". Ratified after the
end of the Civil War they (sort of) ensured former slaves citizenship
and the right to vote, as well as legal barriers to the return to
slavery.
In practice, of course, it would be another 100 years before Blacks
were really able to vote more-or-less freely and Jim Crow laws as well
as anti-Black customs, ubiquitous in both "The North" and "The South",
would prevent these Americans from enjoying even second class
citizenship for many decades to come.