Leaders across the Atlantic expressed sympathies after Tuesday’s terrorist attack in New York, but few could offer advice about how to stop this form of violence. (Christian Hartmann/Reuters)

Across Europe, the deadly truck attack in Lower Manhattan seemed instantly familiar, the latest in a long string of similar vehicle-as-weapon strikes that have left authorities struggling over ways to balance increased security and the traditional free flow of urban life.

Early Wednesday, President Trump appeared to cast the Manhattan attack as a European import, tweeting about retired U.S. intelligence officer Anthony Shaffer's statements on a talk show criticizing Sen. Charles E. Schumer (D-N.Y.). " 'Senator Chuck Schumer helping to import Europes problems,' " he tweeted, citing Shaffer, in an apparent reference to a 1990 visa measure supported by Schumer and signed by President George H.W. Bush. Trump wrote: "We will stop this craziness!"

But as leaders across the Atlantic expressed their sympathies, few could offer concrete recommendations about how to prevent this increasingly prevalent form of terrorist violence.

French President Emmanuel Macron expressed his "emotion and the solidarity of France for New York City and the U.S." In Barcelona, struck in August by an Islamic State-inspired van attack that killed 13, leaders took a brief respite from the Catalan secession battle to "send our solidarity and support to NY, our sister city," in the words of Barcelona's mayor, Ada Colau, on Twitter.

But those expressions of solidarity did not include advice.

In the past two years, often in quick succession, many of Europe's major urban centers have been targeted by vehicle attacks. In 2016, crowds in Nice, France, and Berlin were targeted in incidents that killed 86 and 12, respectively. This year, Stockholm, Barcelona and London were struck — the British capital on three separate occasions — in a wave of violence that claimed more than 35 lives combined. Other isolated incidents have been reported in France.

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[Why truck attacks are so common in Europe, but not the United States]

After a wave of carnage that has continued even under heightened security regimes — such as France's "state of emergency," portions of which became permanent Wednesday — the consensus among European security analysts is that attacks of this nature may be impossible to prevent.

The vehicle attackers "use such rudimentary weapons precisely because they know they won't be prevented by security forces," said Jean-Charles Brisard, director of the Paris-based Center for the Analysis of Terrorism. "On some level, it's impossible — you cannot keep every rental agency under surveillance."

But Brisard added that there are certain measures that cities can take, regardless of macro security policy imposed by governments.

Last month, the European Commission released a set of guidelines designed "to better defend EU citizens against terrorist threats and deliver a Europe that protects."

"As terrorist tactics change, we are stepping up our support to member states in meeting these threats: helping protect the public spaces where people gather, while cutting off terrorists' access to dangerous bomb-making materials and sources of finance," Julian King, the European Commission's chief security officer, said in a statement.

For the most part, these European efforts will include funding — as much as 100 million euros ($116 million) to be invested in city security initiatives. There will also be a forum for cities to exchange their "best practices."

Immediate responses to this new form of terrorist violence have focused mostly on erecting physical barriers to better protect pedestrians.

In Paris this year, city officials announced their plans to extend the Eiffel Tower's security perimeter, a proposal that would add public gardens on the popular landmark's eastern and western sides, as well as a "bulletproof fence" on its other sides.

[How harsh counterterrorism measures can play into the hands of the Islamic State]

At the Cannes Film Festival held each May, strong metal barriers and sturdy planters were installed this year at the entrances to a busy seaside promenade often packed with pedestrians and tourists. The same was true at France's popular Avignon theater festival, where Israeli-made vehicle barriers were used to secure entry areas.

In the Nice attack, by far the deadliest of Europe's recent vehicle assaults, a 31-year-old Tunisian national, Mohamed Lahouaiej-Bouhlel, drove a truck down the city's famed Promenade des Anglais, where crowds were gathered to celebrate France's Bastille Day national holiday. That a truck could enter a nominally secured area caused an enormous outcry in France, where inquiries into police bureaucracy often blamed a complicated system of national and local squadrons that did not coordinate sufficiently.

Much the same has been true in Britain, where London Mayor Sadiq Khan immediately condemned the New York attack. He said in a statement Tuesday that "London stands in grief and solidarity with the great city of New York tonight after the despicable and cowardly terrorist attack in Manhattan."

Khan's statement stood in direct contrast to Trump's comments in June, when, in the immediate aftermath of a vehicle attack on the London Bridge area, Trump wrote on Twitter that Khan's call for Londoners not to be alarmed constituted a "pathetic excuse."

In the wake of the vehicle attacks in London this year, the British capital ramped up its security with the installation of barriers on its bridges, two of which were targeted earlier this year.

In the London Bridge attack, three men drove a van into pedestrians before they got out and began stabbing patrons at a nearby market. In other cities around Britain, local authorities plan to bolster security at Christmas markets by installing obstructions in pedestrian areas. Similar measures were rolled out around the country last year following the December attack in Berlin, where 12 people died in an assault on a popular holiday market.Raffaello Pantucci, director of international security studies at RUSI, a London-based think tank, said the range of places where security barriers are being considered has significantly increased.

The barriers were installed on three bridges: Waterloo, Westminster and Lambeth.

British security analysts said those relatively small changes represent a significant shift.

"Previously, some street market wouldn't get anything," Pantucci said. "Now it does get some attention, and you do see people thinking about putting up barriers or blocking direct access from the street."

Pantucci also noted that police and security services are reaching out to car rental companies "to get onto their radar that this is a possibility and to keep attentive as best you can." Several of the attackers in Europe have used rented vehicles.

Toby Poston, a spokesman for the British Vehicle Rental and Leasing Association, said the industry is also working with authorities to explore "sharing customer information at an early enough stage that can be cross-referenced with their counterterrorism watch lists."

In Germany, however, the focus has been less on physical barriers than on the question of why a particular assailant, such as the 24-year-old Anis Amri, responsible for the Berlin attack, had not been deported or apprehended.

Amri had been slated to be sent back to his native Tunisia, but his return was delayed for bureaucratic reasons. He had also been under surveillance as a suspected radical Islamist who dabbled in drug dealing. But he had dropped off intelligence services' radar.

A federal inquiry by a retired prosecutor released last month was scathing in its assessment of law enforcement's handling of the case.

"Gross mistakes were made that should never have happened," said the leader of the inquiry, Bruno Jost.

Immediately after the attack, authorities vowed to be more aggressive in deporting those who failed to secure asylum in Germany. But those efforts have been hindered, primarily because countries of origin for would-be deportees have resisted taking them back.

Adam reported from London. Griff Witte in Berlin contributed to this report.

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