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Hundreds of thousands of people are protesting in the streets. Rioters are looting stores, burning buildings, and attacking police stations. The authorities have cracked down even as opposition politicians criticize the government for failing to quell the discord.

That describes what’s been happening in France for the past month as students, activists, and labor unions protest everything from education funding to inflation. But it also describes what happened in the U.S. in the 1960s, when protests over racism, the Vietnam War, and the assassination of Martin Luther King Jr. sometimes escalated into riots. Civil unrest can be justified and has sometimes prompted progress. But when protests boil over into violence, history shows that right-wing politicians often benefit.

In 1968, a broad sense of societal disorder after eight years of Democratic leadership helped Richard Nixon win the White House. Now, the riots in France threaten to help Marine Le Pen, who leads the far-right National Rally party, take over the Élysée Palace in next year’s presidential election. Le Pen, who wants to restrict legal immigration, ban Muslims from wearing hijabs, and distance France from the European Union, has been doing her best to turn the unrest to her advantage. “Our country is caught in a vise, held hostage between violence and powerlessness,” she said this week. “In six months, the French people will be able to choose a different path—one of authority and restored security.”

It’s not just France and the U.S. either. In the 1920s, brawls between Italian Blackshirts and workers helped Benito Mussolini position his fascist movement as restorers of order. In 2013, deadly clashes between Hindus and Muslims in the........

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