every era has its touchstone photograph. The Marines hoisting the American flag on Iwo Jima telegraphs the hard-fought triumphs of World War II. The New York City firemen raising the Stars and Stripes over the rubble at Ground Zero after 9/11 captures the resilience of a besieged city. The signature image of Boston’s busing crisis also features the national symbol. It isn’t a heroic one.

On April 5, 1976, Ted Landsmark was running late to a City Hall meeting on minority hiring in the construction industry. Entering the plaza, the African-American lawyer ran smack into the hornets’ nest of an anti-busing rally. A white mob attacked, kicking and beating him. Then Joseph Rakes, a young high school dropout, lunged forward with his weapon of choice.

“I was just out there walking to City Hall in my three-piece suit. I was anyone,” Landsmark told a reporter at the time. Suddenly, someone tried to “kill me with the American flag.”

That quote comes from The Soiling of Old Glory: The Story of a Photograph That Shocked America, published in April by Bloomsbury Press. Author Louis P. Masur revisits this tale of two men, a photographer, and their city in this slim, affecting volume. Entwining history and art, he goes beyond the headlines of Boston’s failed school busing experiment to assess the country’s complex relationship with its national symbol.

“It was an unprecedented act of desecration, one that transgressed every principle most Americans hold dear,” writes Masur, a professor of “American institutions and values” at Hartford’s Trinity College. The Page One photo, coming two years after a federal judge ordered the desegregation of the city’s public schools and four months into the country’s Bicentennial, helped seal Boston’s reputation as the Little Rock of the North.

It almost didn’t happen. Not being eager to cover another anti-busing demonstration, Stanley Forman, a Herald American (now the Boston Herald) photographer, also got a late start that day. As the drama unfolded, his camera’s motor drive froze. Using his manual setting, he still “managed about 23 clicks.” Students of photography will relish such details, as well as Masur’s capsule overview of seminal images in American photographic history.

“We carry historical and visual memories and associations to every image that we see,” he writes. As he points out, the photograph recalls not only Paul Revere’s print engraving of the Boston Massacre, but an 1856 lithograph that spotlights Crispus Attucks, the first victim of the fighting. The parallels did not escape Landsmark and others in 1976.

But Masur doesn’t stop at technique, preferring to delve deeper into the flag’s place in the American psyche. Unpacking the “cult of the American flag” takes the author through a complex historical thicket. The flag as totem gained hold during the Civil War, when the term “Old Glory” came into popular usage. By the end of the 19th century, state legislatures began to adopt anti-desecration laws as the flag was exploited for commercial purposes.

Flag burning also became a popular tool to express political dissent, particularly in wartime. As Masur writes, “Through the 1960s and into the 1970s, the flag had been displayed, waved, erected, decaled, flushed, torn, and burned.” The veneration of Old Glory came full circle after 9/11, when virtually every American became a patriotic flag-waver overnight.

Predictably, African-American attitudes about the flag have veered sharply from ambivalence to outright contempt. Yet Masur notes that “civil rights activists realized that they needed to enlist the flag in their cause if ever they were going to feel represented by it.” That makes the assault on Landsmark all the more stunning.

masur does a serviceable job of tracing Boston’s race relations from colonial times through the earliest struggles over education in the 1840s and 1850s. After World War I, Boston did not experience the great wave of black migration other northern cities did, but its schools were racially mixed, and Masur calls the racial climate of that time “comparatively progressive.” That changed after World War II, when blacks moved to the city in greater numbers. The accompanying rise in segregation and discrimination provided the tinder for the later racial eruptions.

The neighborhood turmoil from South Boston to Roxbury that Masur describes in his book is familiar territory for many local readers. However, his account of how much the attack on Landsmark roiled the State House underscores just how inflamed passions had become by 1976. The House Republican whip, Melrose’s William Robinson, introduced a resolution condemning everyone in spitting distance of the assault except Landsmark. A firestorm ensued, and Robinson’s Democratic counterpart, Everett’s George Keverian, offered an amendment to strike out the sections of the resolution lambasting the passive bystanders, the reporters, the police, and the mayor (Kevin White, who saw the assault from his City Hall office window). Keverian’s amendment was adopted, and the stripped-down resolution passed 216-0. (Twenty-two members did not cast a vote.)

Where Masur excels is not so much in the retelling of these episodes, but with poignant portraits of Landsmark, Forman, and Rakes. Now the president of the Boston Architectural College, Landsmark assumes a Job-like presence throughout the book. Despite being forever linked to a “20-second moment,” he built a successful career interwoven into the city’s political and cultural fabric. Today in his office, alongside the 1856 lithograph of Attucks’s martyrdom, hangs a print of The Soiling of Old Glory.

Forman won his second Pulitzer Prize in two years for the Landsmark photograph, and a third followed. He could have moved on to greener pastures (The New York Times wooed him), but he stayed put in Boston, making the switch to video journalism when he took a job at WCVB-TV in 1983.

And what of Rakes? The celebrated photo has dogged him every step of his life. Shortly before the 25th anniversary of the assault in 2001, a repentant Rakes met with his black coworkers, members of a Big Dig construction crew, to prepare them for what they would see in the press. After each man told Rakes how he felt, an older Jamaican man finally opined, “Shit happens.” His response sums up how far Boston has come.

Gabrielle covers several beats, including mass transit, municipal government, child welfare, and energy and the environment. Her recent articles have explored municipal hiring practices in Pittsfield,...