The white policeman stands in the center of the photograph, the German shepherd’s leash wound loosely around his left hand. With the right, the officer is reaching out to grab the cardigan of the young black protester, drawing him closer to the dog snapping viciously at his waist. The teenager’s eyes are cast down, a living symbol of nonviolence, his knee thrust forward as if to block the attack. Behind him on the street, other African Americans look on with alarm.

The Associated Press photo, taken on May 3, 1963, as Eugene “Bull” Connor’s Birmingham, Ala., police force trained water hoses and snarling dogs on peaceful civil rights protesters, startled and shamed white newspaper readers across the country.

“The image of the savage attack struck like lightning in the American mind,” civil rights historian Taylor Branch wrote. Two days later in a White House meeting with liberals, President John F. Kennedy fumed in frustration at the photograph splashed above the fold on the front page of the New York Times and declared civil rights “a national crisis.”

The image symbolized the brutality of American racism, so it resonated deeply when President Trump conjured it Saturday to warn crowds protesting the death of George Floyd, the 46-year-old black man who died after a white Minneapolis police officer pressed a knee into his neck.

If protesters had attempted to breach the White House fence Friday, Trump tweeted, “they would have been greeted with the most vicious dogs, and most ominous weapons, I have ever seen. That’s when people would have been really badly hurt, at least.”

The remarks were met with outrage in many quarters, and a scathing reply from D.C. Mayor Muriel E. Bowser (D), who said the president had “glorified violence” by recalling some of the worst images in civil rights history.