One hundred fifty-one years ago today, a warship landed in the harbor of Galveston, Texas. Union general Gordon Granger was on board, along with about two thousand Union soldiers.

Later that day, Granger stood on the balcony of Ashton Villa — Galveston’s most luxurious mansion and a onetime Confederate military headquarters, built by slaves just five years earlier. He announced the dawn of a new era, reading from General Order 3, a document declaring the end of the Civil War and reasserting the power of the Federal government over the soon-to-be reconstructed South.

But Granger’s most important message was directed not to Galveston’s cotton planters or shipping magnates, but to their slaves: