While movements as a whole are propelled by long-term vision and deep-seated grievances, mass protests in particular emerge in response to what some theorists call “trigger events.” Here, a dramatic incident such as a natural disaster, a media exposé, a political scandal, or a killing may ignite passions and prompt outrage, sending people into the streets. A moment of unrest may extend for weeks or even months, especially when guided skillfully by organizers.

But activists trying to sustain momentum face serious headwinds. The media are fickle in their attention, and a flitting spotlight makes it hard for activists to be seen and to draw in new people. Already, although issues of racial justice raised by the protest remain a crucial part of public discussion, headlines have moved away from focus on street actions and instead turned to stories such as President Donald Trump’s reelection rallies and the surge in new coronavirus cases.

Another factor that can slow protests is that they demand a serious commitment of time and energy from participants; eventually, many tire out. As the weeks stretch on, the pressures of ordinary life begin to reassert themselves, and people must return to responsibilities that they might have set aside in a moment of crisis. Some of those who are most committed may be encumbered with arrest and subsequent, time-consuming legal entanglements. As just one example, protests at the 2000 Republican National Convention, in Philadelphia, resulted in hundreds of arrests, bail as high as $1 million for those deemed “ringleaders,” and a legal battle that took years to resolve.