Just after 3 p.m. on that fateful day, President Lincoln heard a commotion outside his White House office, followed by a woman’s cry.

“But I need to see Mr. Lincoln!”

Lincoln opened the office door and saw the guards hustling the woman out. The date was April 14, 1865, exactly 150 years ago.

“I’m here,” he reportedly said. “Tell me what you want. I have time for all good people who need to see me. Let the good woman come in.”

Lincoln invited the woman into his office. She proceeded to tell him that she was Nancy Bushrod. She said she and her husband, Tom, had been slaves at the Harwood Plantation outside Richmond until they were freed by the Emancipation Proclamation. Her husband was off serving with the Union Army of the Potomac while she raised their three children—twin boys and a baby girl—in Washington, D.C. ...