Amendment XV

Section 1. The right of citizens of the United States to vote shall not be denied or abridged by the United States or by any State on account of race, color, or previous condition of servitude.

Section 2. The Congress shall have power to enforce this article by appropriate legislation.

The Fifteenth Amendment was ratified February 3, 1870.

From Nate Segal:

We all know how this amendment came about after the Civil War to give the right to vote to the former slaves.
What I didn't know was that African-Americans had been denied voting rights in the North, also.

Lawrence M. Friedman wrote:

In 1860, there were only five states, all in New England, which permitted blacks to vote. Massachusetts was the only state to allow black men on juries. The post-Civil War amendments gave Northern blacks the vote, but did not change very much the climate of opinion.

– A History of American Law, Second Edition (New York: Simon & Schuster, 1985) page 507.

"The climate of opinion" is, of course racism. Laws don't change what's in people's hearts. Within only several years after this amendment was ratified, Southerners devised ways to deprive African American's the vote and, in fact, virtually all civil liberites.

You might say that the North won the Civil War, but the South won the peace.