August 20, 2015 5:49 AM EDT

The meeting was tense from the start. Huddled behind a blue curtain at a redbrick middle school in Keene, N.H., members of the Black Lives Matter movement pressed Hillary Clinton on her role in promoting the tough-on-crime policies of her husband’s Administration. “What in your heart has changed?” Julius Jones, an organizer with the group’s Worcester, Mass., chapter, asked the presidential candidate and former Secretary of State. “How could those mistakes that you made be lessons for all of America?”

Few voters win a private audience with the Democratic front runner. Fewer still would use the moment to criticize her. But the exchange with Clinton, held in a spare room after an Aug. 11 campaign stop, was meant to be difficult. One of the guiding principles of the protest movement that has come to be known as Black Lives Matter is that discomfort can bring change. Activists have spent the year since Michael Brown’s death in Ferguson, Mo., working to make the rest of the country confront racial issues that many would rather ignore.

That has meant shutting down highways in St. Louis, holding die-ins in New York City and Washington, blocking bridges in Charleston, S.C., and protesting at police-commission meetings in Los Angeles. Lately it has meant disrupting presidential campaign events in Phoenix and Seattle. Black Lives Matter has staged more than 1,000 demonstrations, rallying everywhere from Texas to Tel Aviv to keep the spotlight trained on the effects of structural racism. “We’re competing for attention,” says Samuel Sinyangwe, 25, one of the movement’s many emerging leaders, “with everything else that’s going on in the world.”

The young movement has already won some notable victories. In response to the uprising, the White House convened a new task force on policing. Criminal-justice reform bills have found bipartisan support in Congress. Each of the major Democratic campaigns has held meetings with Black Lives Matter activists to solicit ideas. And the nation has taken notice. In a Pew Research Center poll released in August, 59% of respondents said the U.S. must do more to achieve racial equality, a 13-point jump in a single year.

“I don’t think we knew that it was going to sweep the country and the world in the way that it has,” says Alicia Garza, one of three co-founders of the Black Lives Matter network. “But we’re just getting started.”

The movement began on social media. In July 2013, George Zimmerman was acquitted of murdering Trayvon Martin, an unarmed black teen shot to death during a struggle in a gated community in Sanford, Fla. Garza, an activist with the National Domestic Workers Alliance in Oakland, Calif., was sitting in a bar when the verdict came down. She posted a missive on Facebook, punctuated by a powerful sentiment: “Black people. I love you. I love us. Our lives matter.”

Her friend Patrisse Cullors, another veteran California activist, who founded the prison-reform organization Dignity & Power Now, replied with a message that included the hashtag #blacklivesmatter. It gained a modest foothold in the year after Zimmerman’s acquittal. But after Brown’s death, the hashtag became a rallying cry. On the November day a St. Louis grand jury declined to indict Darren Wilson, the Ferguson cop who shot Brown, the hashtag was used 202,492 times, according to Twitter data compiled for TIME.

By then, Black Lives Matter had become popular shorthand for a broader array of causes. The original group founded by Garza, Cullors and a New York–based immigration activist, Opal Tometi, now counts 26 chapters, including foreign outposts in Toronto and Accra, Ghana. (Organizers must subscribe to a shared “set of principles” to start a new branch, Garza says.) But the formal network is just one of many organizations that have converged under the Black Lives Matter banner to confront the power structures–from police forces to prisons to politics–that activists say have devalued black life.

The movement comprises a broad coalition. Many of its followers are women; many are gay; some are transgender; some aren’t black. It has no formal leadership and no shortage of leaders. A prominent cohort emerged from Ferguson, including Brittany Packnett, a 30-year-old educator from St. Louis who became a part of the White House task force on police reform, and DeRay Mckesson, who quit his job as a school administrator in Minneapolis to devote himself to the cause full time.

The millennials steering the movement have a strong sense of history. “We’re standing on the shoulders of giants that came before us,” says Charlene Carruthers, 30, a Chicago activist involved in causes ranging from helping black youth to raising the minimum wage. But they’re keen to exploit the technology of today. “We have different tools at our disposal. We have the power of social media. We have the advantage of retrospect. And many of us have been organizing for a long time.”

Success breeds a new set of challenges. Like other recent protest movements, such as the Tea Party and Occupy Wall Street, Black Lives Matter is devoted to its decentralized power structure. “We refuse to create a hierarchy of issues,” says Tia Oso, an Arizona native and veteran activist who in July disrupted a forum with Democratic candidates Bernie Sanders and Martin O’Malley. That model has been a strength, allowing protesters from a range of ideologies to unite under a common banner. But it may hinder the process of developing a clear platform.

The diversity of goals and grievances was on display in late July, when some 1,500 activists gathered in Cleveland for an event billed as the first national convening of the Movement for Black Lives. There were workshops and speeches on topics ranging from conflict resolution to feminism, political organizing to drug decriminalization, mindfulness to hip-hop music. The danger, says Deana Rohlinger, a Florida State University sociologist who studies protest movements, is “that you’re so decentralized that you don’t have a unified message.” And without formal leadership, there is no one deputized to determine who gets to speak on its behalf.

One way the movement can exercise power is through presidential politics. Activists are warning Democrats not to take black votes for granted in 2016. “We are going to have very clear demands,” says Packnett. “If those aren’t met … people may not show up to vote.” In response to criticism from Black Lives Matter, Sanders outlined a new racial-justice platform and hired a black activist to serve as a spokesperson. O’Malley called for a constitutional amendment to protect voting rights and unveiled a detailed criminal-justice program that calls for body cameras, national use-of-force standards, eliminating mandatory-minimum sentences for low-level drug offenses and better data collection on police shootings.

Clinton has also echoed the movement’s mantra on the campaign trail. During the meeting in New Hampshire, she urged the activists to develop a set of defined political goals. “Your analysis is totally fair,” she told them. “But you’re going to have to come together as a movement and say, Here’s what we want done about it.”

Jones bristled. He called the suggestion “a form of victim blaming.” The activist wanted a response from the heart; the politician offered pragmatic advice. It was the kind of raw exchange that has infused the movement with power, illustrating both how far it has come and how far it has yet to go.

–With reporting by DANIEL WHITE

This appears in the August 31, 2015 issue of TIME.

Write to Alex Altman at alex_altman@timemagazine.com.

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