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Seventy-five years ago today, at a naval munitions base northeast of San Francisco called Port Chicago, 320 men were killed in a massive explosion that “filled the sky with flame,” destroyed two ships and shattered windows miles away.

It also tipped off a chain of events that would pave the way for the desegregation of the U.S. armed forces.

Because the Navy was segregated and the U.S. was in the throes of World War II, the men doing the dangerous work of loading munitions onto ships were almost all black and inadequately trained.

“I said, ‘Lieutenant, one of these days, this base is going to blow sky high,’” one survivor, Joe Small, recalled later. “He said, ‘Well, if it does, you won’t know anything about it.’”