Armed police respond outside Parliament during an incident on Westminster Bridge in London, Britain March 22, 2017. Stefan Wermuth / Reuters

If, as police suspect, the deadly attack near the British Parliament on Wednesday proves to be an act of terrorism, it will depart from the recent pattern of terrorist attacks in the West. In the post-9/11 world of the counterterrorism surveillance state and the internet-radicalized lone-wolf attacker, terrorists typically don’t use sophisticated weapons that might tip off authorities, and it’s easiest not to strike at heavily fortified targets. Instead, they often use easily accessible, relatively crude weapons (knives, guns, vehicles) against “soft” targets (nightclubs, Christmas markets, crowds celebrating Bastille Day).

After the 2015 Paris attacks, for example, The Washington Post marveled, “There was no pretense of attacking nodes of the power structure. They didn’t try to blow up a naval vessel, an embassy, a military barracks. They did not attack government buildings or police stations. The killers went after people having fun—dining out on a Friday night, going to a concert or watching a ‘friendly’ between France and Germany at the soccer stadium.”

In the case of the violence outside Westminster Palace on Wednesday, the weapons appear to have been crude—a car, a knife—but the target was one of the hardest in the nation. The result was not casualties on the scale of the Paris attacks—the authorities appear to have responded swiftly to the incident before an attacker could get into Parliament itself—but it was devastating nonetheless: lawmakers placed on lockdown and evacuated to Westminster Abbey, the British prime minister spirited away in a silver Jaguar, and several people, including a police officer, lying dead or injured just outside the seat of government.

While terrorists usually attack poorly defended targets, the terrorism scholar Benjamin Cole writes, a “key feature of terrorism is that terrorist groups will continually innovate in order to defeat defences around specific targets.” And those targets include government facilities, since terrorists aim in part to “undermine the principal foundation of the state”—the “perceived invincibility that it cultivates amongst its people”—and to “demonstrate the failure of the state to protect its key leaders and strategic installations.” (Government entities are the third-most common target of terrorist attacks around the world since 1970, after private citizens/property and and businesses.)

As Bruce Hoffman, a terrorism expert at Georgetown University, has noted, while the Provisional Irish Republican Army’s attacks against the British leaders Margaret Thatcher and John Major in the 1980s and ’90s technically failed, they “shone renewed media attention on the terrorists, their cause, and their impressive ability to strike at the nerve center of the British government even at a time of heightened security.” The IRA, one Northern Irish police officer observed at the time, is “always that step ahead of you.” The terrorist groups and their motivations have changed since then, but that enduring fact is one reason terrorism remains so difficult to prevent.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.