“This time they said, ‘We’re not going.’ ”

That’s how Seymour Pine of the New York Police Department’s Morals Division described the raid he led on the Stonewall Inn in New York’s Greenwich Village on June 28, 1969, and the unprecedented refusal of the gay men in the bar to hang their heads in shame and go silently into the paddy wagons. The “Stonewall riots” that resulted are generally regarded as the beginning of the gay rights movement in the United States. A documentary featuring Pine and other participants, Stonewall Uprising, aired on PBS’s American Experience last week. (It played in independent theaters in 2010; DVDs are available.)

Pine’s comment made me think about resistance to oppression. Sometimes all it takes is one person or a few people saying, “We’re not going” to light the spark of a movement or a revolution.

Fourteen years earlier Rosa Parks had sat down on a Montgomery, Alabama, bus. When the bus driver told her to stand up to give her seat to a white passenger, she said, “I don’t think I should have to stand up.” Later in her autobiography she wrote:

People always say that I didn’t give up my seat because I was tired, but that isn’t true. I was not tired physically, or no more tired than I usually was at the end of a working day. I was not old, although some people have an image of me as being old then. I was forty-two. No, the only tired I was, was tired of giving in.

More recently Mohammed Bouazizi, a vegetable seller in Tunisia, got tired of the corruption, contempt, and confiscation he faced at the hands of local officials. On December 17, 2010, he went to provincial headquarters and set himself on fire, dying on January 4.

Rosa Parks helped to galvanize a civil rights revolution that changed America in less than a generation. The Stonewall uprising created a gay rights movement that also brought revolutionary change, even though gay people are still working toward equal rights. And Time magazine called Bouazizi “The Man Who Set Himself and Tunisia on Fire” — though that article appeared before the fire spread across the Arab world.

To these, of course, one could add many others: the American farmers who fired the shot heard round the world at Concord Bridge, the Gdansk shipyard workers who launched Solidarity in 1980 or the Leipzig peace marchers in 1989.

Over the centuries many other people have risked their lives to stand up against oppression without successfully rallying others or sparking a revolution. Some make an impact, like the Hungarian Revolution of 1956, but there are many more whose names are lost.

What causes some acts of resistance to succeed? Is it historical inevitability, just the right moment for the dry field of hidden dissatisfaction to be set on fire by a spark? Some libertarian — and other — radicals wonder why Americans don’t revolt against what the radicals see as tyranny.

In Foreign Affairs, Jack Goldstone posits the factors that lead to revolution:

The necessary and sufficient conditions are fourfold: a government that has come to appear so irremediably unjust or inept that it is widely viewed as a threat to the country’s future; elites (especially military) that are alienated from the state and no longer willing to defend it; broad-based popular mobilization that spans different groups and classes; and international powers that are unwilling to step in to defend a threatened regime, or that constrain it from using maximum force to defend itself.

He’s talking about the overthrow of a ruler or regime, of course, not just a major change like the civil rights revolution. But some of the same factors seem relevant, including a status quo that has come to seem irremediably unjust, a change in the perspective of some elites, and broad-based mobilization. A key factor seems to be lack of democracy, either in the polity as a whole or for a particular group. The United States was a democratic country in the 1950s, but not so much for black people. Especially in the South, they were effectively prevented from voting or otherwise participating in normal politics. And indeed in the Deep South they lived under many of the conditions of a police state. Gay people could vote, but they were forced to deny the reality of their lives to family, friends, employers, coworkers, neighbors, and strangers on the street. As historian Eric Marcus says in Stonewall Uprising, “Before Stonewall, there was no such thing as coming out or being out. The very idea of being out, it was ludicrous. People talk about being in and out now, there was no out, there was just in.” William Eskridge, professor of law at Yale and author of Gaylaw: Challenging the Apartheid of the Closet (and also the author of the Cato Institute’s brief in the Lawrence v. Texas case, cited in the Supreme Court’s opinion), says, “The federal government would fire you, school boards would fire you. So you couldn’t have a license to practice law, you couldn’t be a licensed doctor. You needed a license even to be a beautician and that could be either denied or taken away from you.”

It’s not always clear when the “tipping point” of popular discontent has been reached, but social media are making it easier for the alienated to find allies. If they find enough traction, all that’s needed is a spark — and an act of individual courage.