Sunday, May 6, marks the fifth anniversary of the assassination of Pim Fortuyn. Fortuyn was nine days away from an election from which he was expected to emerge as Dutch prime minister. As he walked out of a radio studio near Amsterdam, a left-wing activist named Volker van der Graaf pumped five bullets into his back. Fortuyn died almost instantly.

The killer would later explain that Fortuyn’s views on Muslim immigration made him a “danger.” It was the Netherlands’s first political assassination in over 300 years.

Fortuyn had been an active politician for only a few months but had already shaken things up dramatically. Before him, Dutch politics had been essentially a closed club whose members shared broadly similar views on major issues and abhorred open conflict.

Then along came Fortuyn, a writer and sociology professor who’d grown increasingly concerned about the rapid Muslim influx into the Netherlands — and about the fact that while the Dutch government lavishly subsidized immigrant families, schools, mosques, and community centers, it made little effort to integrate newcomers and refused to challenge the patriarchal, often brutal values that held sway in Muslim enclaves.

Fortuyn recognized the rise of fundamentalist Islam in Europe as a menace to democracy. And he said it straight out — eloquently, forcefully, fearlessly. Back in 1997 he’d published one of the first books anywhere to sound the alarm. Only days before September 11, 2001, he wrote that communism’s role as a threat to Western freedom “has been taken over by Islam.”

But instead of recognizing him as a prophet, Dutch leaders saw him as a threat. On September 11, Dutch Moroccans gathered in the streets to cheer. But the interior minister, Zaken De Vries, ignoring these enemies within, warned instead that counterintelligence services would “pay sharp attention to persons who want to … conduct a cold war against Islam.” Meaning Fortuyn.

In November 2001, Fortuyn became head of a new party, Livable Netherlands, only to be tossed out three months later for being too outspoken. So he started another party. The more he spoke out, the more journalists and politicians smeared him — an openly gay man and life-long liberal — as a right-wing extremist, a racist, a new Mussolini or Hitler.

Yet millions of his countrymen knew better. Accustomed to leaders who shunned controversy and spoke in empty formulas, Dutchmen were stunned and delighted to hear Fortuyn say things they’d long been thinking themselves. Voters from all over the political map became his ardent supporters. He seemed poised not only to transform the Netherlands but also to lead the way for all of Western Europe.

And then, suddenly, he was dead. Van der Graaf’s explanation of his motives read like a précis of every lie that had ever been told about Fortuyn. Dutch citizens were justifiably outraged at the journalists and politicians who’d told those lies. Feeling the heat, the Dutch parliament reformed immigration law — to an extent. It overhauled integration policies — somewhat.

Leading the way in advocating these policy changes were two admirers of Fortuyn’s — filmmaker Theo van Gogh and Parliament member Ayaan Hirsi Ali. But by early 2007 they, too, were out of the picture.

In November 2004, an Islamist murdered van Gogh. In 2006, in a crisis that brought down the government, Ms. Hirsi Ali was hounded out of Parliament by colleagues desperate to unload this troublemaker. When she moved to Washington, D.C., last year, polls showed that many Dutchmen wouldn’t miss her. The elite, it seemed, had reasserted its power, and the Dutch people, tired of conflict, had embraced the status quo ante.

This was confirmed by the March 2006 elections, in which immigration — incredibly — was a minor issue. Five years ago, Fortuyn inspired widespread hope and determination. Today, all too many Dutch citizens seem confused, fearful, and resigned to gradual Islamization. No wonder many of them — especially the young and educated — are emigrating to places like Canada, Australia, and New Zealand.

Yes, some politicians, notably Parliament member Geert Wilders, are carrying on Fortuyn’s battle. But momentum has given way to malaise. Politicians and journalists who once kept mum on Islamization now openly defend it as preferable to culture clash: Amsterdam mayor Job Cohen has called for “accommodation with the Muslims,” including toleration “of orthodox Muslims who consciously discriminate against their women.”

Only last week, Mr. Wilders was called in by Dutch intelligence and security officials who, he said, “intimidated” him by pressuring him to tone down his rhetoric on Islam. Fortuyn’s brief shining moment seems very long ago.

Many political assassinations leave behind haunting questions. How would Reconstruction have gone under Lincoln? Could the Vietnam debacle have been avoided if President Kennedy had lived? Five years after Fortuyn’s murder, it can feel as if Volkert van der Graaf robbed Europe not only of a brilliant champion of liberty, but of its one great chance to save itself before it’s too late.

Mr. Bawer is the author of “While Europe Slept” and lives in Oslo, Norway.