So opined Olivier Roy, arguably Europe’s foremost authority on political Islam, in an essay published days after Hosni Mubarak was forced from power in February. “Look at those involved in the uprisings, and it is clear that we are dealing with a post-Islamist generation,” he wrote. “This is not to say that the demonstrators are secular; but they are operating in a secular political space, and they do not see in Islam an ideology capable of creating a better world.”

Mr. Roy wasn’t alone in the sangfroid department. “I am not in the least bit worried about the Muslim Brotherhoods in Jordan or Egypt hijacking the future,” confided New York Times columnist Tom Friedman, with the caveat that their secular opponents would need some time to organize. Added his colleague Nicholas Kristof in a dispatch from Cairo: “I agree that the Muslim Brotherhood would not be a good ruler of Egypt, but that point of view also seems to be shared by most Egyptians.”

What reassurance. Nine months on, the Islamist Nahda party has swept to victory in Tunisia, the one Arab state in which secularist values were said to be irreversibly fixed. Libya’s new interim leader, Mustafa Abdel-Jalil, came to office promising “the Islamic religion as the core of our new government”; as a first order of business, he promises to revoke the Gadhafi regime’s ban on polygamy since “the law is contrary to Shariah and must be stopped.” Later this month, Islamist candidates—some of them Muslim Brothers, others even more religiously extreme—will likely sweep Egypt’s parliamentary elections.

It doesn’t stop there. Hezbollah has effectively ruled Lebanon since it forced the collapse of a pro-Western government in January. Recep Tayyip Erdogan, Turkey’s Islamist prime minister, cruised to a third term in parliamentary elections in June. Hamas, winner in the last vote held by the Palestinian Authority in 2006, would almost certainly win again if Palestinian President Mahmoud Abbas dared put his government to an electoral test.

Getty ImagesWhen secular politics fail, Islamism is the last big idea standing.

Why have Islamists been the main beneficiaries of Muslim democracy? None of the usual explanations really suffices. Islamists are said to be the unintended beneficiaries of the repression they endured under autocratic secular regimes. True up to a point. But why then have their secular opponents in places like Egypt been steadily losing ground since the Mubarak regime fell by the wayside? Alternatively, we are told that secular values never had the chance to sink deep roots in Muslim-majority countries. Also true up to a point. But how then Tunisia or Turkey—to say nothing of the Palestinians, who until the early 1990s were often described as the most secularized Arab society?

Closer to the mark is Mideast scholar Bernard Lewis, who noted in an April interview with the Journal that “freedom” is fairly novel as a political concept in the Arab world. “In the Muslim tradition,” Mr. Lewis noted, “justice is the standard” of good government—and the very thing the ancien regimes in Tunisia, Egypt and Libya so flagrantly traduced. Little wonder, then, that Mr. Erdogan’s AK party stands for “Justice and Development,” the Egyptian Muslim Brotherhood’s new party is “Freedom and Justice” and, further afield, the leading Islamist party in Indonesia calls itself “Prosperous Justice.”

Still, the Islamists’ claim to “justice” goes only so far to account for their electoral successes. There is also the comprehensive failure of the Muslim world’s secular movements to provide a better form of politics.

The national-socialist brew imported from Europe in the 1940s by Michel Aflaq became the Baathist tyrannies of present-day Syria and Saddam Hussein’s Iraq. Pan-Arabism’s appeal faded well before the death of its principal champion, Gamal Abdel Nasser.

Socialism failed Algeria; Gadhafi’s “Third Universal Theory” failed Libya. French-style laïcité descended into kleptocracy in Tunisia and quasi-military control in Turkey. Periodic attempts at market liberalization yielded dividends in places like Bahrain and Dubai but were never joined by political liberalization and were often shot through with cronyism.

That sour history leaves Islamism as the last big idea standing—and standing at a moment when tens of millions of young Muslims find themselves undereducated, semi- or unemployed, and uniquely receptive to a world view with deep historic roots and heroic ambitions.

What does its future hold?

Optimists say it need not be a reprise of Iran; that it could look more like Turkey; that the term “moderate Islamist” isn’t an oxymoron, at least in a relative sense. Then again, Turkey’s domestic and foreign policies inspire little confidence that moderate Islamism will be anything other than moderately repressive and moderately radical. As for Iran, signs of its own long-awaited turn toward moderation are as fleeting as the Yeti’s footsteps in drifting snow.

The good news is that after 31 years most Iranians have grown sick of Islam always being the answer, and the collapse of the regime awaits only the next ripe opportunity. The bad news is that a similar time-frame may be in store for the rest of the Muslim world, until it too becomes disenchanted with Islamist promises. Get ready for a long winter.

Write to bstephens@wsj.com