On a Saturday in January in downtown San Francisco, half a block up from Market Street, shoppers and business people witnessed a jarring sight: a huge red swastika flag hanging over O'Farrell Street.

A crowd began to gather under the flag, eventually numbering as many as 3,000, The Chronicle reported. Enraged onlookers chanted, "Take it down! Take it down!" A nearby department store draped another flag beside the 4-by-8-foot Nazi banner – an even bigger, 10-by-20-foot American flag. But that wasn't enough to pacify what police were soon calling a riot.

Finally, two sailors on leave – Harold Sturtevant Jr. and E.G. Lackey – set about scaling the building's fire escape. Three other men and Chronicle photographer Bill Young trailed after them. Sturtevant ducked into the 10th floor window, followed by Lackey, who handed him a knife and lowered him onto the flagpole.

The date was Jan. 18, 1941. About 11 months later, Japan would launch an attack on the Pearl Harbor naval base in Hawaii, prompting American involvement in World War II. But up until that point, the United States had been walking a fine line with Germany, maintaining diplomatic relations with the country despite growing popular resentment toward its fascist regime. It was not to last.

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The ninth floor of 26 O'Farrell St., now one story below a jewelry shop, was then home to a German consulate. The flag was being flown to mark a national anniversary, and the consulate was legally allowed to display it. Sturtevant would later claim that he knew none of this; he assumed the flag was owned by a Nazi supporter. The crowd was "beginning to get mean," Sturtevant recounted to The Chronicle. He simply "thought it would be a good thing to get that swastika out of sight."

Sturtevant found himself sitting astride the flagpole, nine stories up. He reached for the flag. A consulate staffer tried to wrest it from his grasp, and a brief tug-of-war ensued, but Sturtevant prevailed. He tore the Nazi flag apart while the crowd roared its approval. Some grappled over the resultant shreds as they fluttered to the ground, hoping to keep them as souvenirs.

The mood in the streets was jubilant. But the "flag incident," as it was termed, instantly made national headlines and sparked a high-stakes diplomatic row. Consul General Fritz Wiedemann called the defacement of the Nazi flag a violation of international law. "The facts speak for themselves," he said in a statement.

Wiedemann happened to live in the picturesque Whittier Mansion in Pacific Heights, as SFGATE reporter Bob Bragman details here.

Secretary of State Cordell Hull expressed the U.S.'s formal regrets, even after Berlin condemned the flag-slashing as typical of a lawless democracy. "Even in the jungle, the rights of guests are respected," one dispatch seethed. Ultimately, the owner of the building terminated the consulate's lease, and the Germans vacated their O'Farrell outpost.

Local public opinion was as disparate as any social media firestorm today. "That U.S. sailor who tore down the Nazi flag ... was tearing at the very roots of what I interpret as civil liberties," The Chronicle's Herb Caen wrote. Several letters sent to The Chronicle's editor defended the two seamen, while others derided them as reckless. "If this world had more men of the Harold Sturtevant type," one letter said, "Hitlerism and Nazism would never have had a ghost of a chance."

Lackey and Sturtevant, for their part, were arrested and found guilty of misconduct. But the arresting officer testified that the men "felt they were doing their duty" in destroying the flag. Without a dishonorable discharge on his record, Sturtevant later re-enlisted.