Raised by firefighters from the smoking wreckage of the World Trade Center, a flag telegraphed both anguish and resolve. Planted atop a mountain on Iwo Jima, another piece of national cloth came to symbolize collective perseverance and conquest.

Around the globe, flags — some of nations, others of affiliation — have wrapped spectators at soccer matches and participants in protest marches, flown over revolutions and holy wars, adorned advertisements and marked lunar landings. But even to people gazing up at the same flag, it can mean very different things. And, experts say, there may be nowhere else in the world where flags stir more intense feelings than in the United States.

That was proven again after a massacre at a historic black church in Charleston, South Carolina, touched off fevered criticism of the Confederate flag, with politicians who had long tiptoed around questions about its meaning suddenly calling for it to be removed from the statehouse grounds.

The man police charged with the attack, Dylann Roof, posted photos online showing him burning a U.S. flag and holding a Confederate flag, along with a manifesto laying out hatred of minorities. In another photo, he wore a jacket bearing the flags of apartheid-era South Africa and white-ruled Rhodesia.

"Flags are, by their nature, emotion-charged emblems, and that's especially the case in the U.S.," says John Hartvigsen, president of the North American Vexillological Association, a group of scholars dedicated to the study of flags. "What does the flag mean? Well, who's looking at it? And that's the whole issue with the Confederate flag."

The notion of flags as potent symbols is hardly new or exclusively American. Roman legions carried banners into battle. In Nazi Germany, the flag emblazoned with a swastika came to embody an ideology now so loathed that modern-day Germany bars it display. In Iraq and Syria, masked members of the Islamic State group have seized control of cities under their own black-and-white banner.

But in the U.S., particularly since the Civil War, when soldiers leading troops into battle were shot out from under the banners they carried, flags have come to embody ideology and stir passions in ways that have few modern international equivalents, experts say.

"We are unique in the extent and depth of our worship of the flag. There's no nation on earth like us," says Rick Shenkman, associate history professor at George Mason University and editor of the History News Network.

Marc Leepson, author of "Flag: An American Biography," agrees. "We don't have a monarch or a state religion," he says. "In some ways, the flag is a substitute."

Leepson recalls that when he was writing his book, he solicited online comments from people around the world on how they regarded their nations' flags. The response, he said, was almost unanimous. "They said, 'We love our flag but nothing like you Americans do.' .... People are as patriotic as Americans are. They just don't have this deep emotional attachment."

But often that attachment seems to overlook the ambiguity of flags' meaning.

"The thing about the flag is that it's not language. People use language to invest it with meaning but because it's not language itself, it's for everybody to say what they think it means," said Carolyn Marvin, a professor of communication at the University of Pennsylvania and author of "Blood Sacrifice and the Nation: Totem Rituals and the American Flag."

Leepson points out the many ways the Stars and Stripes are honored: The annual celebration of Flag Day (June 14), the Pledge of Allegiance recited daily by schoolchildren, the singing of "The Star Spangled Banner" in so many venues, including countless Fourth of July celebrations.

Yet, during the 1960s, the same flag was burned by Vietnam War protesters to signal their disagreement with U.S. foreign policy. And it's that same flag, going back more than a century, that has been used on commercial packaging of products like whiskey.

"You're not going to drive around England and see Union Jacks (the United Kingdom's flag) displayed around car dealerships," Leepson says. "That's an American thing."

The Confederate flag, too, has its own long history of widely differing interpretation.

Over the decades, it has been adopted by some as a symbol of Southern heritage, even as others decried it as an emblem of slavery and hate. Marvin calls it as an "undigested piece of American history."

"It is a totem and a memorial to the Confederate soldier. It's a cloth testament to their service and to their descendants. When it's attacked as a symbol, that's essentially seen as a condemnation of their families and themselves," said John Coski, author of "The Confederate Battle Flag: America's Most Embattled Emblem."

Many have come to embrace it as a symbol of rebelliousness and a kind of good ol' boy mentality portrayed in "The Dukes of Hazzard," the former TV series; to others, it's regarded as a stark reminder of Jim Crow. But Coski, a historian at the American Civil War Museum, believes it is too powerful to simply disappear.

"There are still people who are dedicated to it and devoted to it," he says. "In these controversies, when one side pushes to remove something, the other side always pushes back."

The passions that both the U.S. and Confederate flags arouse in Americans contrast with the place of flags overseas.

In Japan, the administration of Prime Minister Shinzo Abe, concerned about the national flag's lack of prominence, this year issued a directive strongly encouraging the country's universities to fly it. But the flag's long-ago association with the country's wartime imperialism leaves some uncomfortable.

In Romania, demonstrations in 1989 that presaged the fall of dictator Nicolae Ceausescu saw protesters assert independence by cutting holes in a national flag to remove its Communist insignia.

But in the U.S., where immigration has created a population of widely disparate backgrounds, flags have lasting and far-reaching currency.

Flags can "act as communal 'umbrellas' under which people with vastly different views can gather and unite — whether physically or in spirit — without having to explore the different meanings that the flag in question might have for each of them," Richard Jenkins, a retired professor at England's University of Sheffield and co-editor of "Flag, Nation and Symbolism in Europe and America," said in an email.

That was evident in the proliferation of American flags in every corner of the U.S. following the Sept. 11, 2001, terrorist bombing.

But Hartvigsen, the flag scholar, says flags' power — to unite or divide — is only as great as the significance people assign to them. That is at least as true today, he said, as 101 years ago when U.S. Interior Secretary Franklin Lane delivered a speech, describing a "conversation" with a living Stars and Stripes.

The flag, he said, told him: "I am no more than what you believe me to be and I am all that you believe I can be. I am what you make me; nothing more."