For all the years of worry that terrorists would develop more sophisticated methods, Thursday’s attack in Nice realized a fear that turned out to be far worse: a form of violence so crude that it renders many of our usual defenses useless.

The attacker, in ramming a plain white truck through crowds of holiday revelers, killed 84 people and remade an everyday vehicle, a familiar sight on streets around the world, into an object of menace and fear.

No group has claimed the attack. The attacker’s motivation remains unknown, as do questions of whether he acted alone.

And yet this act, whatever its particulars, represents the culmination of long-building trends, in which terror tactics become more rudimentary and the targets more random. It is forcing a recognition that security and intelligence measures, long the core of Western thinking, are of limited utility and can never provide total safety from an individual who decides to kill.

This is shifting pressure onto more abstract and unproven counterterrorism methods that do not promise to halt violence but merely ameliorate underlying political or social drivers. And it is straining the politics of Western countries, where leaders have spent the past 15 years describing terrorism as a war that could be won.

The populations targeted by terrorism, from Turkey to France to the United States, are confronting a difficult new reality, in which the danger can be managed or policed but perhaps never entirely overcome.

A different sort of problem

The attacks on Sept. 11, 2001, established a narrative that has shaped Western thinking on terrorism ever since, even as many analysts consider that view obsolete.

The enemy, whether al-Qaida or someone else, became seen as a cohesive group that operated from far away. That enemy coordinated complex, military-style assaults against a subset of targets that, while civilian, held symbolic or political value. Governments could anticipate and defend the targets, monitor or disrupt the groups, and ultimately defeat them by targeting their leaders and seizing their territory.

But the threat has evolved into something much harder to counter. Insurgent groups in mid-2000s Iraq, unable to defeat the U.S.-led occupation force, instead targeted Iraqi civilians wherever they gathered, establishing a method that terror groups now deploy internationally.

In 2008, Pakistani militants killed 166 people in Mumbai, India, attacking what experts call “soft” targets: places such as hotels and train stations that are populated but, because of their seeming randomness, rarely defended. At security conferences in Western capitals, officials and analysts began to worry about whether they could prevent a “Mumbai-style attack” in their own countries.

Then came the rise of “lone wolf” attackers who acted on their own, without training from or often even contact with the terrorist groups they claimed to serve. Attacks are planned within the minds of individuals whose intentions remain hidden until the shooting begins.

Such attacks predate the Islamic State, though the group emphasizes them, disseminating propaganda that provides tactical guidance and ideological justification available to anyone with an internet connection.

The use of a truck in Nice was new only in the specifics and in the degree to which it has forced a realization increasingly difficult to ignore: In the world of lone wolves and Mumbai-style attacks, more barricades and metal detectors and monitoring programs can improve security, but can’t guarantee it absolutely.

What will it take?

As terrorism itself becomes harder to prevent or defend against, policy experts say governments should instead address two deeper but murkier problems: the ideology that calls for such violence and, even harder, the social and political factors that drive an individual to accept that ideology.

But how do you defeat an ideology? Destroying the Islamic State’s “caliphate” in Iraq and Syria, some argue, would undercut its grandiose claims and dampen its appeal. It would also limit the group’s ability to coordinate large-scale attacks, which in turn help inspire lone actors who may, or may not, be driven by ideology.

Still, even as the group loses territory, its propaganda would survive online. And the problems that contributed to that ideology’s rise in the Middle East would remain: authoritarian states that often use violence themselves, religious institutions often seen as complicit, stifled economies that grant young people few opportunities, and a cycle of conflict that encourages tribalism and extremism.

This is why many analysts are urging Western governments to look at their own populations and ask not whether terrorist ideologies can be eradicated, but why individual citizens might become susceptible to messages telling them to kill on behalf of a foreign terrorist group.

“It’s a social and political problem,” said Peter Neumann, a professor at King’s College London and the director of the International Center for the Study of Radicalization.

Neumann said that, increasingly, Western citizens who turn to terrorism “are not deeply ideological or even religious.”

Rather, such individuals often seem to come to terrorism as a way to bring meaning to their lives or to explain their own sense of helplessness or isolation.

Muslim or migrant populations have struggled to integrate into Western European countries, often left feeling excluded from national identities that stress secularism and are rooted in specifically European heritage.

“Ghettoization, polarization, and alienation in a country like France are profound,” Neumann said. “Bottom line is: it’s about feeling like you belong.”

Western governments have tried to address this by providing greater welfare programs to disadvantaged communities or, often, by more forcefully policing them.

But these, Neumann said, risk deepening the polarization between communities, which can be bridged only by building an “inclusive sense of national identity.” European societies have resisted this, instead giving rise to right-wing politics that emphasize ethnic and religious heritage.

“I think this will be with us for a long time, sadly,” he said.

A political question without an answer

Since Sept. 20, 2001, the day that President George W. Bush declared a “war on terror,” Western leaders have used the terminology of war both to explain the threat and to articulate their response.

“It is a war against terrorism, against jihadism, against radical Islam,” Manuel Valls, the French prime minister, said after a terrorist attack outside Paris in January 2015.

A decade and a half of war has not delivered Western populations to safety, highlighting the gap between rhetoric and reality. But politicians have been constrained by this language, unable to acknowledge that, while war is a struggle that pulls a nation together and can eventually be won, the realities of terrorism can only be managed and the threats, at some point, merely endured.

But political leaders, compelled to promise an answer to this week’s violence in Nice, returned to the familiar formulations.

“We have an enemy that will continue to strike all people, all countries that have liberties as essential values,” France’s president, Francois Hollande, said.

Hillary Clinton told CNN on Thursday that “we are at war with these terrorist groups and what they represent.” Though she urged enhanced intelligence gathering to combat terrorism, she has previously called for more aggressive use of force against the Islamic State in Syria and Iraq.

Donald Trump, in an interview with Fox News, said he would ask Congress to declare war on terrorism.

While this framing might reassure, it raises a question: Why does the war never feel as if it is approaching victory?

This question’s unanswerability perhaps contributes to the fear that is increasingly destabilizing Western politics. And it creates space for right-wing populists, rising throughout Europe, to present more aggressive policies as the answer.

“Nothing that we have proposed has been put in place,” Marine Le Pen, who leads the right-wing party National Front, told the French newspaper Le Figaro, calling for measures to revoke citizens’ nationality, close certain mosques and ban organizations.

Valls, the French prime minister, perhaps sought to break this cycle Friday when he said, “The times have changed, and France is going to have to live with terrorism.”