On Sunday, just hours after three men launched an assault on London Bridge, British Prime Minister Theresa May stepped in front of 10 Downing Street and told the world, “We believe we are experiencing a new trend in the threat we face.” In many ways, the attack in the British capital, as well as others over the past two years in Nice, Berlin, Stockholm, Paris, and Manchester, actually weren’t all that unique in terms of tactics, targets, or even motive. A century ago, a battered horse-drawn wagon loaded with a hundred pounds of dynamite—attached to five hundred pounds of cast-iron weights—rolled onto Wall Street during lunch hour. The wagon stopped at the busiest corner in front of J. P. Morgan’s bank. At 12:01 P.M., it exploded, spraying lethal shrapnel and bits of horse as high as the thirty-fourth floor of the Equitable Building, on Broadway. A streetcar was derailed a block away. Thirty-eight people were killed; many were messengers, stenographers, clerks, and brokers who were simply on the street at the wrong time—what are today known as “soft targets.” Another hundred and forty-three people were injured.

That attack, on September 16, 1920, was, at the time, the deadliest act of terrorism in American history. Few surpassed it for the next seventy-five years, until the Oklahoma City bombing, in 1995, and then the September 11th attacks, in 2001. The Wall Street case was never solved, although the investigation strongly pointed to followers of a charismatic Italian anarchist named Luigi Galleani. Like ISIS and its extremist cohorts today, they advocated violence and insurrection against Western democracies and justified innocent deaths to achieve it.

Europe has also faced periods of more frequent terrorism than in the recent attacks. Between 1970 and 2015, more than ten thousand people were killed in over eighteen thousand attacks, according to the University of Maryland’s Global Terrorism Database. The deadliest decades were, by far, the nineteen-seventies and eighties—during the era of Germany’s Baader-Meinhof gang, Italy’s Red Brigades, Spain’s E.T.A., Britain’s Irish Republican Army, and others. The frequency of attacks across Europe reached as high as ten a week. In 1980, I covered what was then the deadliest terrorist attack in Europe since the Second World War, when a bomb, planted in a suitcase, blew up in the waiting room of Bologna’s train station. Eighty-five people were killed; body parts were everywhere. A neo-fascist group, the Armed Revolutionary Nuclei, claimed credit.

Yet May is correct: modern terrorism is still evolving. It has already gone through distinct phases, with shifting missions, messages, and means of mobilizing. The onset is generally associated with the early acts by radical Palestinian groups in the late nineteen-sixties, such as the 1968 hijacking of an El Al flight from Rome to Tel Aviv. A half century later, terrorism is now a standard feature of asymmetric warfare, with fewer wars pitting states against each other and more of the combatants being non-state actors with less traditional forms of weaponry. One of the most striking trends is the way professional or experienced terrorists are being supplemented by a proliferating array of amateurs, Bruce Hoffman, the author of the classic “Inside Terrorism” and director of security studies at Georgetown University, told me.

“There may have been, in aggregate, more terrorism in the seventies and eighties, but it was discriminate,” he said. “They kept their terrorism within boundaries related to their cause. Today it’s different. It’s less predictable, less coherent and less cohesive. It leaves the impression of serendipity. ISIS posts pictures of a vehicle and says get in your car and drive into people—and that’s all it takes.”

Another major difference in the early twenty-first century is that the most salient movement is a transnational religious movement, which is a stark departure from the secular Marxist or nationalist cells in the seventies and eighties, according to William Braniff, the executive director of the University of Maryland’s National Consortium for the Study of Terrorism and Responses to Terrorism. “Those groups do not carry the same polarizing feature that religious extremism allows—that they are God’s people and others are damned,” he told me. They are more often willing to kill.

So, while the absolute number of attacks is down, the lethality of terrorism has risen sharply in the past two years, Braniff said. Between 1970 and 2014, there were no fatalities in fifty-three per cent of terrorist attacks worldwide. In 2015, the number of lethal attacks increased by eight per cent. The number of people killed in each lethal attack also increased.

Jihadi extremism has evolved through its own phases—and motives, goals, locations, and tactics—since the first generation fought the Soviet Union’s occupation of Afghanistan, between 1979 and 1989. The number of jihadis has grown exponentially with each mobilization, according to Clint Watts, a former F.B.I. counterterrorism specialist who is now at the Foreign Policy Research Institute, in Philadelphia. The time required to “swarm”—or gather in an arena—has been roughly halved with each generation. Courtesy of both social media and recruitment by earlier militants, the latest crop of jihadis fighting with ISIS is drawn from a wider assortment of nations that are often further afield. Most ominously, each generation is also more extreme in its ideology and ambitions.

Today’s third generation is engaged in plots that are simpler yet more widespread than the 9/11 attacks by Al Qaeda, Watts told me. “They’re not as sophisticated as in the Al Qaeda era, when complex operations were well coördinated and carried out by a few designated men. Now, some are not even trained or formally recruited. They’re self-empowered.” As a result, killing people on a bridge may not have the same impact or symbolic emphasis as an attack on a U.S. Embassy or the World Trade Center. But the reaction can be just as profound.

The indiscriminate nature of terrorism today makes it ever harder to contain, Hoffman, of Georgetown, noted. “Thirty or forty years ago, terrorists did not have the ability to overwhelm authorities. With lone wolves today, law enforcement is often flying blind.”

“It’s very difficult to see how open liberal democratic societies can counter a threat that is much more individualistic, like the attacks in Britain, and that have the feeling of spontaneity,” he added.

The twin attacks in Britain come as the Islamic State is close to defeat in Iraq and under growing pressure in Syria, where its capital, in Raqqa, is surrounded. Last week, Syrian rebels said that a new offensive into Raqqa was imminent. Tens of thousands of ISIS foreign fighters have been killed since the separate campaigns, both backed by U.S.-led airpower, were launched late last year. The movement’s credibility, which was based on running its own state, has been sapped.

Yet the collapse of the Islamic State’s caliphate could pose new threats to the West, J. M. Berger, a fellow at the International Center for Counter-Terrorism—the Hague, told me. “The jihadi movement is fragmenting. There’s big change happening—with ISIS and its ability to hold territory and with what happens to jihadis. It will be very difficult for us to stay ahead of them.”

ISIS propaganda—in online publications, audio messages, on social media, and the encrypted Telegram messaging service–has been urging followers and sympathizers to stay away and instead wreak havoc at home. Its slick publication Rumiyah (which is Arabic for “Rome”) offers graphic instructions for the kinds of attacks witnessed on London Bridge and in the other European onslaughts.

“The West can do things on the margins to be safer,” Berger said, but it still faces another “five or ten years of potentially dangerous situations. There’s not any silver bullet that will reduce the occurrence of these events in the short term. We need to be thinking about resilience—and how we’re going to assimilate events when they happen.”