After terrorists murdered nearly 3,000 people on Sept. 11, 2001, Americans swiftly responded with fellowship and patriotism.

People of different races and religions consoled one another at candlelight vigils. Others tattooed stars and stripes on their bodies. American flags rose up in neighborhoods across the nation. On the evening of the attacks, roughly 150 members of Congress — Democrats and Republicans — stood shoulder-to-shoulder and sang God Bless America on the Capitol steps.

“America is united,” proclaimed President George W. Bush the day after the tragedy.

Matt Kindsvogel, a Navy veteran stationed in Saudi Arabia on 9/11, says he returned home a few months later to a county awash in red, white and blue. “People were actually nice to one another," he says. "It didn’t matter what color you were on the outside, you were an American and that is all that mattered.”

Today, much of that cohesion has crumbled. Americans fight bitterly about everything from gun control to Obamacare. Disagreements over the positions of presidential candidates Hillary Clinton and Donald Trump have marred relationships, with heated discussions on Facebook, Twitter and other social media sites helping fuel the divide. At the same time, the country has seen extreme examples of malice and prejudice toward fellow Americans, including racist graffiti and rallies put on by white nationalist groups.

In USA TODAY Network interviews with more than two dozen Americans across the political spectrum, as well as replies to our network’s social media queries, a clear yearning for more unity emerged. Yet Americans struggle with how to get there in a world of partisan politics, vicious social media interactions, blustering pundits and aggressive media coverage of even such trivial issues as first lady Melania Trump's shoe choices.

Even our feelings of pride in America are vastly incongruous. In the year after the attacks, more than nine in 10 Democrats and Republicans said they felt "extremely” or “very” proud to be an American, according to a Gallup poll. This year, Republican and Democrat feelings on that topic were vastly different, at 92% and 67% respectively, marking the largest-ever gap in the 17 years since Gallup has taken the pride survey.

The U.S. “is much more divided this 9/11 anniversary than ever before,” says Johanna Perez, 28, an engineering student from Miami. “Politics have divided people firmly. So much so that we don't see each other as Americans as a whole but as 'them' vs 'us'.”

How we got here:

There are myriad reasons for the transformation from American unity to discord.

Americans sparred over how to best deploy a war on terror, arguing over the benefits of invading Iraq and implementing the Patriot Act. At the same time, the number of media channels and partisan pundits proliferated, producing a steady stream of soundbites that often enthralled or enraged.

Social media gave Americans a megaphone to share disparate views. People argued in person and online about blue lives, black lives, Syrian refugees, transgender bathrooms and Colin Kaepernick's refusal to stand for the national anthem.

A contentious and protracted primary season put people on edge, then tensions escalated further as voters went head-to-head over who would make the best president.

Consider:

Four in 10 people have argued with family or friends about the election, and just over one in 10 have completely cut off ties with someone, according to a Reuters/Ipsos poll.

About half of social media users say political conversations on social media are less respectful and more hostile than discussions they’d have on other venues, according to survey data from the Pew Research Center.

More:Hear directly from Americans

Americans vs. Americans

After the 9/11 attacks, the nation rallied against a common enemy. It was Americans vs. the terrorists.

But soon the country was back to Americans vs. Americans.

In some cases, the fear associated with the attacks stoked division, says Ziad Munson, an associate professor of sociology at Lehigh University in Bethlehem, Pa.

There was prejudice and “scapegoating" in which "people face real fears about various issues in their lives and they find people, or very often groups of people, to blame," he says. Muslims, for instance, "suffered from that in the aftermath of the 9/11 attacks."

Munson also points to national discord before 9/11.

"The divide after 9/11 had already started, and was growing, before the 9/11 attacks," he says.

Prior to 9/11, the country's conflicts were apparent. The 2000 George W. Bush vs. Al Gore presidential election, for instance, was a messy nail-biter with a vote recount in Florida. Bush prevailed, but like Trump, he didn’t win the popular vote.

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Disunity expected

All the disharmony shouldn’t be a surprise, says Wayne Fields, former director of American culture studies at Washington University in St. Louis.

What we forget is “how radical the very concept of America is,” he says. “From the very start, we were this conglomeration. We came from lots of different cultures and lots of different backgrounds."

Fields initially saw digital advances as “a hope for more unified, informed citizenship.” Yet there's often more declarations than public discourse, he says.

"What it’s done in lots of ways is to narrow our vision, to put us into smaller and smaller groups of like-minded people.” he says.

Arnold Gluck, a rabbi in Hillsborough, N.J., has similar concerns. "We tend to exist in echo chambers and we listen to people who think like we do and don’t listen to other opinions," he says.

Gluck's other worry: That people are addicted to inflammatory content. "If you can get somebody riled up, they are going to pay attention," he says. "It’s like a sugar high or a drug. ... Once you’ve got people hooked, you can just keep feeding it."

Fight nicely

Robust discussion, and even disagreement, can add to a healthy democracy, says Fields. “Division of ideas is crucial," he says.

What matters is how someone presents his or her case, and how they move forward after a dispute, says Lynn Bufka, a psychologist who helps the American Psychological Association develop its Stress in America survey.

"Smart people disagree all the time on what the best outcome is for different things," she says.

It comes down to "respect other people’s opinions. Treat people the way you want to be treated," says former New York City Fire commissioner Salvatore Cassano.

Cassano, promoted to FDNY chief of operations after 9/11, says he sees examples of division and the dark sides in the nation, yet has faith in the country's ability to come together.

“When things are tough, we unite,” he says, pointing out the Hurricane Harvey relief efforts. “It reminds me of what happened after 9/11, after Katrina, after Sandy.”

Veteran Kindsvogel — who has "9-11" tattooed on the knuckles of his right hand — also says the goodwill following Hurricane Harvey is a sign of hope, but adds that the nation needs to unite more often when there isn't a crisis.

He suggests "instead of lashing out at each other because we are different from each other, learn from each other."

"America is beautiful because of the diversity that is here," he says. "Don’t make our differences the reason that America falls."

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