On July 17, 1944, hundreds of sailors were loading ammunition onto two cargo ships in Port Chicago, Calif., not far from San Francisco, when an explosion powerful enough to be felt 50 miles away killed 320 of the men, most of them Black.

More than 400 sailors were injured in the blast, the cause of which has never fully been determined.

When ordered to continue loading ammunition the next day, 258 Black sailors objected until safety conditions improved. All of them were subjected to a sham trial and convicted of various offenses, though most of them eventually agreed to return to work at the piers. The situation was so bereft of justice that Thurgood Marshall, who was then a lawyer for the N.A.A.C.P. and would later became a Supreme Court justice, attempted to intervene on their behalf.

The group of 50 men who continued to resist were given dishonorable discharges and jail sentences. They became known as the Port Chicago 50, and their case was used as a driver in the early days of the civil rights movement nationwide and helped lead to the desegregation of the armed forces.

On Wednesday, 80 years after the explosion at Port Chicago, the Navy secretary, Carlos Del Toro, officially exonerated all 258 Black sailors, none of whom are still alive.

“Today, the U.S. Navy is righting a historic wrong,” President Biden said in a statement after the announcement.