If you want to understand the power of the images that crowd our 21st-century media, look to the words of a famous 19th-century crook.

When William Marcy Tweed ran New York’s Tammany Hall in the post-Civil War years, reformers and good-government types railed against his ability to drain millions of dollars out of the city’s treasury—all to little effect. But when cartoonist Thomas Nast began drawing biting cartoons for Harper’s Weekly, he drew blood.

“Stop them damned pictures!” Tweed demanded. “I don’t care so much what the papers say about me. My constituents don’t know how to read, but they can’t help seeing them damned pictures!”

Today, in matters great and small—from the video of a professional athlete beating his fiancée, to the ghastly beheadings of journalists at the hands of ISIS—Tweed’s words still have resonance. The old adage that “a picture is worth a thousand words” has never seemed more relevant. Indeed, the power of the image has helped to shape our history. The question today is whether “the picture” may be so powerful that it overrides critical judgments about what we are seeing.

There’s no doubt about how images helped shape America’s response to two of the most significant matters of the recent past: civil rights and Vietnam.

The depredations suffered by Southern blacks at the hands of white authorities had been known, at an abstract level, for decades. But when Birmingham police set police dogs and fire hoses against protesting teenagers in the spring of 1963, the images captured by network TV crews and by photographers Charlie Moore (for LIFE Magazine) and Bill Hudson (for the AP) hit with special force. President John Kennedy, who had been a reluctant warrior in the civil rights efforts, said they were "so much more eloquently reported by the news camera than by any number of explanatory words.” Three months later, he went on national TV to proclaim that civil rights was “primarily a moral issue”—the first time any American president had uttered such sentiments.

Two years later, when Americans watched on their television screens as Alabama police savagely beat marchers on the way from Selma to Montgomery, it took barely a week for Lyndon Johnson to appear before a joint session of Congress and declare that “We shall overcome.” The swift passage of the 1965 Voting Rights Act was due in no small part to those images.

The Vietnam War offers textbook cases about what pictures can do to shape our understanding. On August 5, 1965, CBS News aired a story by Morley Safer about a company of Marines that burned the thatched roof huts in the village of Cam Me with flamethrowers and Zippo lighters. The images were so devastating that, as Safer noted years later, the authorities tried to claim it had only been a “training exercise” on a mock village. (President Johnson himself called a CBS executive to assert that CBS had “shat on the American flag”). To Americans who had seen only the images of World War II that army censors had permitted, the torching of Cam Me seemed to symbolize a conflict where we were no longer the good guys.

Less than three years later, when the Tet Offensive sent Viet Cong and North Vietnamese into virtually every South Vietnamese city, photographers captured the street corner execution of a Viet Cong officer at the hands of the chief of the Vietnamese National Police. (The Viet Cong officer had just murdered a South Vietnamese colonel along with his wife and six children—something photographer Eddie Adams noted in arguing that his photo had been mistakenly used as a symbol by antiwar advocates.)

Which brings us to the images that compel our attention today.

As has been noted repeatedly, there was no doubt about what Ray Rice did to his then-fiancée in that Atlantic City casino elevator; he told police and league officials what he’d done. The videotape added nothing new to our knowledge, but it added mightily to our emotional knowledge. With that tape, we “knew” what had happened in a far more visceral, powerful way than words alone could convey. In that sense, the video was like the images of starving refugees in Ethiopia captured by the BBC’s Michael Buerk in October of 1984—images that led to a worldwide relief effort (and the first Live Aid concerts). Read a newspaper story and you shake your head and reach for the coffee; see the pictures and you want to write a check, or call someone to offer help.

But can a picture prove—too much? Consider what we know and do not know about ISIS. There’s a deep division among intelligence agencies and others about just how big a threat to the United States this group is. (See, for example, this Washington Post story.)

But there’s no doubt about what those pictures show: If ISIS has the power to seize and to murder American and British citizens, that seems to suggest a clear and present danger. Maybe they can slip across the Mexican border, bearing the Ebola virus; maybe they can commandeer commercial airliners, or dispatch suicide bombers into Times Square.

The emotional knowledge we have about ISIS may have little or nothing to do with real knowledge. But in the scales of influence, there’s little doubt about what carries more weight. Whatever the intelligence does or does not say, we “know” that ISIS is a menace requiring American action, with all of the potential risks that the Law of Unintended Consequences may bring.

There’s an old story—almost certainly false—that publisher William Randolph Hearst, told by a correspondent in Havana in 1898 that “there will be no war,” responded, “You furnish the pictures, I’ll furnish the war.” More than a century later, there’s cause to wonder whether the power of the picture will turn that legend into reality.