Bernie Sanders’s insurgent presidential campaign has opened up a debate about how social change happens in our society. The official version of how progress is won—currently voiced by mainstream pundits and members of a spooked Democratic Party establishment—goes something like this: politics is a tricky business, gains coming through the work of pragmatic insiders who know how to maneuver within the system. In order to get things done, you have to play the game, be realistic, and accept the established limits of debate in Washington, DC.

A recent article in The Atlantic summed up this perspective with the tagline, “At this polarized moment, it’s incremental change or nothing.” This view, however, leaves out a critical driver of social transformation. It fails to account for what might be the most important engine of progress: grassroots movements by citizens demanding change.

Social change is seldom either as incremental or as predictable as many insiders suggest. Every once in a while, an outburst of resistance seems to break open a world of possibility, creating unforeseen opportunities for transformation. Indeed, according to that leading theorist of disruptive power, Frances Fox Piven, the “great moments of equalizing reform in American political history”—securing labor rights, expanding the vote, or creating a social safety net—have been directly related to surges of widespread defiance.

Unlike elected officials who preoccupy themselves with policies considered practical and attainable within the political climate of the moment, social movements change the political weather. They turn issues and demands considered both unrealistic and politically inconvenient into matters that can no longer be ignored; they succeed, that is, by championing the impractical.

Such movements, of course, face immense barriers, but that shouldn’t stop us from acknowledging their importance and highlighting the key role played by moments of mass defiance in shaping our world. Outbreaks of hope and determined impracticality provide an important rebuttal to the politics of accommodation, to the idea that the minor tweaking of the status quo is the best we can expect in our lifetimes.

Here, then, are three moments when the world broke open—and two when it still might.

Civil Rights: An “Unwise and Untimely” Movement

In hindsight, it’s easy enough for people today to imagine that progress on civil rights was preordained. But that’s hardly how things looked as the 1960s began. Six years after the Supreme Court’s 1954 Brown v. Board of Education ruling declared “separate educational facilities…inherently unequal,” defiance of the law had become a badge of honor for officials throughout the South. White Citizens’ Councils had come to dominate local politics in much of the region, and ever more vocally racist politicians were winning elections to Congress over more genteel (if still bigoted) Southern politicians of a previous generation.

Civil-rights bills had passed in Washington, DC, in 1957 and 1960, but only after they were watered down to homeopathic levels. Activists even debated whether to ask President Dwight Eisenhower to veto the first of those bills, and Thurgood Marshall deemed the second “not worth the paper it’s written on.” However inadequate those bills were, Eisenhower had expressed doubts that any further legislation would be enacted for at least a decade, possibly two. On taking office, President John Kennedy was hardly more hopeful and possibly even less enthusiastic when it came to taking action of any sort. As journalist Todd Purdum has noted, Kennedy “believed that strong civil rights legislation would be difficult if not impossible to pass, and that it could well jeopardize the rest of his legislative program.”

In this context, civil-rights activists were constantly counseled to embrace the mildest sort of incrementalism and avoid divisive tactics.

Those who reignited the civil-rights movement did so by roundly ignoring this advice. Highly disruptive sit-ins at lunch counters starting in 1960 were followed by the Freedom Rides of 1961, in which interracial groups of activists dramatically endeavored to desegregate interstate busing in the South. These actions commanded public attention and compelled politicians who only wanted to ignore civil-rights issues to take a stand. Gradually, the disruptions began to create a new consensus around the urgency of ending Jim Crow discrimination.

The genius of Martin Luther King Jr. lay in recognizing that the explosive events altering the perceived limits of the possible were not accidents; rather, there was a craft to engineering them. With the Birmingham campaign of 1963, aimed at breaking segregation’s hold on that city, his Southern Christian Leadership Conference teamed up with local activists in a premeditated attempt to create a public crisis, using escalating acts of civil resistance to galvanize popular sentiment.

Liberal critics called the campaign “unwise and untimely.” Yet the gambit of the activists paid off handsomely. As historian Michael Kazin has pointed out, nationally televised scenes of police dogs snapping at unarmed demonstrators and water cannons opening up on student marchers “convinced a plurality of whites, for the first time, to support the cause of black freedom.” Moreover, the effort spawned a wide array of copycat protests. “A score of Birminghams followed the first,” explained the organizer of the Freedom Rides, James Farmer. By some counts, close to 1,000 demonstrations took place across the South during the summer of 1963, resulting in some 20,000 arrests.

“Birmingham, and the protests that immediately followed it,” writes historian Adam Fairclough, “transformed the political climate so that civil rights legislation became feasible; before, it had been impossible.”

When Dictators Fall

Throughout much of the 20th century, it was believed that nonviolent uprisings could not succeed in an authoritarian context—that if Gandhi, for instance, had protested not the British rule of India but the rule of some state like Hitler’s Germany or Stalin’s Soviet Union, he would have promptly disappeared or been murdered. Yet in the past several decades, campaigns of civil resistance have prevailed against a remarkable variety of undemocratic and repressive regimes, from the Philippines to Chile, Poland to Tunisia, in ways that shocked seasoned observers.

One dramatic example was Serbia. In mid-1999, Slobodan Milosevic, the strongman who had ruled the country for a decade and whose campaigns of ethnic cleansing against Bosnian Croats and Muslims had earned him the nickname “the Butcher of the Balkans,” had a solid grip on power. He had survived protests over election theft and had passed laws reining in freedom of the press and the independence of the country’s university system. A 78-day bombing campaign by NATO that year only rallied the population to his nationalist appeals. By August, The Washington Post wrote, Milosevic was “more firmly entrenched than ever.”

Just over a year later, however, he would lose power amid mass protests and shortly thereafter be sent to The Hague to face trial as a war criminal. A crucial catalyst in the upheaval that unseated him was a youth-based group called Otpor.

Otpor’s disobedience began with small, often humorous stunts, designed to show that resistance was possible. For example, when authorities in the city of Novi Sad brought much official pomp to the construction of a new bridge over the Danube River, even though the bridge was little more than a temporary pontoon, Otpor seized the opportunity. The activists ceremoniously built their own toy bridge over a pond in one of the city’s central parks. The stunt left authorities with two bad options: arrest people for creating a Styrofoam prop and look cartoonishly repressive, or let Otpor mock the regime without reprisal.

While attracting amused audiences with such displays, the group’s activists were methodically signing up new recruits for training sessions on the principles of nonviolent revolt. Ultimately, they drew tens of thousands of people into their networks. In the fall of 2000, when Milosevic attempted to steal another election, they were ready. Allying with the country’s labor movement and opposition parties, Otpor helped launch an escalating series of strikes and protests that climaxed when more than half a million people massed in the center of the capital, Belgrade, on October 5, forcing Milosevic to cede power.

“Initially Otpor was viewed as just another student organization with no real political influence, and neither the regime nor the opposition parties paid much attention to it,” former activists Danijela Nenadic and Nenad Belcevic have written. “By the time the regime realized the strength, impact, and significance of Otpor, it was too late to stop the momentum of resistance.”

There can, of course, be limits to what such nonviolent mass revolts can accomplish in situations rife with repression. As Egypt’s experience in the Arab Spring shows, dramatic upheavals are no substitute for long-term organizing that can build alternative institutions and give democratic forces something to fall back on in bad times (as the times certainly are in Egypt under the repressive military government currently ruling the country). Nevertheless, unarmed uprisings have prompted some stunning transformations. Using a database, they compiled of more than 300 struggles against undemocratic regimes, political scientists Erica Chenoweth and Maria Stephan offered striking evidence in 2011 that nonviolent movements can and do succeed with remarkable frequency.

If those fighting in undemocratic societies have an advantage in their pursuit of transformative change, it’s this: there is little pretense that insider politics is a viable route to change of any sort. Left with few good options, those who are unsatisfied with the world they inherited have little choice but to transform unrealistic aims into winning ones.

Gay Marriage: Touching the Third Rail

Lest anyone imagine that transformative change takes place only in faraway places or bygone decades, the struggle around gay marriage provides an example of just such a shift that was recent, swift, and thorough. Unlike Otpor’s antics or the disobedience campaigns of the civil-rights movement, advocates for same-sex marriage did not generally rely on civil disobedience or mass protest to generate momentum. (There were, however, notable exceptions to this, including San Francisco Mayor Gavin Newsom’s 2004 decision to marry same-sex couples in defiance of state law, marches on Washington in 2000 and 2009, large-scale demonstrations in California against controversial Proposition 8, which attempted to permanently ban gay marriage in that state, and a variety of incidents in which members of the clergy broke official prohibitions to perform such weddings.)

Though its focus may not have been mass protest, gay marriage was still a notable instance in which change came not through the leadership of an outspoken president or champions in Congress, but despite all the timid political realists in Washington.

A decade and a half ago, as author and gay-rights activist Marc Solomon has pointed out, “no state allowed same-sex couples to marry, support for gay marriage nationwide hovered around 30%, and politicians everywhere thought of it as the third rail of American politics—draw near at your peril.” Leaders of the movement to make it an American reality like Evan Wolfson, founder of the organization Freedom to Marry, worked long and hard to engender a major shift in public opinion as a way to transform the political weather.

“I’m not in this just to change the law,” Wolfson argued in 2001. “It’s about changing society.” Pushing only for more easily obtainable gains, such as domestic partnerships, he contended, was a mistake. Instead, he advocated going “into the room, asking for what we deserve, telling our powerful stories, and engaging the reachable allies. We may leave the room not getting everything we want, but don’t go in bargaining against yourself.” The movement committed itself, in the words of historian Josh Zeitz, “to a decades-long campaign to win the hearts and minds of ordinary voters.”

Remarkably, the transformation came far more quickly. As state-level campaigns progressed, the once politically toxic issue morphed into a mainstream crusade. In 2011, for the first time, polls showed public support for same-sex marriage to be over 50 percent. After that, progress came at a furious pace as the world broke open. Politicians suddenly began announcing that they had “evolved” on the issue, with six senators declaring their support for same-sex marriage in a single April week in 2013. Hillary Clinton joined them soon after, explicitly announcing her support, despite having declared in 2004, “I believe marriage is not just a bond but a sacred bond between a man and a woman.” Even prominent conservatives began to convert. Former Representative Robert Barr, for one, who had sponsored the Defense of Marriage Act in 1996, came to favor its repeal.

Between 2010 and 2014, more than a dozen states joined the growing list of jurisdictions allowing same-sex marriage. Increasingly, the wins came via legislation and public votes, not merely court decisions by judges. By the time the Supreme Court was ready to make major rulings on the issue, it was hardly a fair fight.

As Wolfson put it, “We had persuaded the country, and the courts followed.”

Breaking Open New Possibilities

The beauty of impractical movements is that they confound established expectations about the political future, which means it’s difficult to predict when and where new outbreaks of defiance and hope will succeed in capturing the public imagination. Still, there are a number of areas in which, at this very moment, activists are developing strategies of disruptive revolt with an eye to producing the kind of whirlwind moments that can redefine the public perception of what’s politically practical and necessary.

Two that might matter in the near future are those focused on immigrant rights and climate change.

One of the great political success stories of the last five years has been the unlikely triumph of the DREAM Act students. These immigrant youths, mostly Latino, advocated a piece of legislation designed to provide legal status for the children of undocumented immigrants: young people brought to the country as kids, who spent their formative years in the United States and are seeking to attend college or serve in the armed forces. Previously, such young people had no choice but to live in the shadows, with the danger of deportation constantly hanging over their families. But like gay-rights activists before them, an emergent movement of DREAMers made “coming out”—in their case, about their undocumented status—a point of pride and political power.

Traffic blockades, civil disobedience at detention centers, and even sit-ins at Obama campaign offices made their cause one that the White House could not ignore. Starting in June 2012, President Obama issued a series of guidelines that would ultimately allow more than 5 million immigrants, including DREAM Act students, to live and work in the country legally. Moreover, immigrant-rights protests have placed Republicans in the unenviable position of either enraging their nativist base or alienating one of the country’s fastest-growing voting blocs. Vox correspondent Dara Lind calls this “the massive prisoner’s dilemma the GOP faces on immigration.”

Now, many leaders from this youth movement are about to launch a major push for a comprehensive immigration solution, one that would be less vulnerable to court challenges or changes of administration than President Obama’s executive orders. Having consciously plumbed lessons from the tradition of civil resistance, they are making an effort sure to be creative, bold, and confrontational.

Another issue on which activists are organizing in a major way and that may be primed for a breakthrough is climate change—and a breakthrough is needed, if civilization as we know it is to continue. It’s frequently said that climate change represents a tougher fight than an issue like same-sex marriage, since the latter involved a change in cultural attitudes but threatened to impose no significant economic costs on giant corporations or economic elites.

It’s true that environmentalists are squaring off against some of the world’s most profitable and powerful companies, like ExxonMobil. But this is hardly the end of the story. Conventional wisdom once held that the British would never leave India because of the profits generated by colonialism and the Raj. Great Britain was then the most powerful force in the world, and many believed that it could never be moved. But that was only until, in a changing world, a transformative local resistance movement rewrote the equation, economic and otherwise.

In the future, severe super-storms, rising sea levels, floods, droughts, record heat waves, spreading wildfires, and other ecological dislocations and disasters resulting from global warming will begin to pile up billions of dollars in lost property, failed crops, tidal waves of refugees, and the like. This means that one day in the not-so-distant future, the economic forces lining up on either side of climate change may not seem so desperately unequal.

Those who have already taken direct action to combat climate change have been told again and again that their efforts would yield few results. And just as frequently they have proved the skeptics wrong. The Keystone XL pipeline, slated to bring carbon-dirty tar sands oil to the U.S. Gulf Coast from Canada—the target of civil disobedience from Washington, DC, to Nebraska to Alberta—was once considered a “done deal” by industry analysts. Canadian Prime Minister Stephen Harper called it a “no brainer,” and Secretary of State Hillary Clinton indicated that she was inclined to sign off on it. In 2010, few would have predicted that President Obama would ultimately veto the project.

The ongoing effort to convince institutions such as churches and universities to divest from fossil fuel companies—a campaign backed by international protests and campus building occupations—has similarly been derided as “misguided” and “at best, completely ineffective.” Yet with investment funds worth $3.4 trillion combined having shifted away from these industries, the Globe and Mail reports that “changing investor attitudes are starting to hit coal-related firms across the globe” as “pension fund managers and other institutional investors are now questioning the long-term returns offered by coal and oil companies.”

A determined generation of climate activists, reared on these struggles, aided by international allies, and engaged with past social movements, is prepared to make fossil fuel extraction a moral issue and fight to alter the limits of political debate in order to offer humanity the hope of a decent and reasonable future. Like others who have broken open the world of possibility, they recognize that there is a cost to playing the inside game, to basing your politics on the Beltway version of hardheaded realism—especially given the grim future realism of global environmental disaster.

Never before has humanity depended so fully for the survival of us all on a social movement being willing to bet on impracticality.