It is the fifth terrorist attack in England in 2017
, coming on top of three incidents in London and one in Manchester
from March to June, with a combined death toll of 36.
Of course, this is still small, relative to the Irish Republican Army campaigns that caused thousands of deaths across Ireland and England through the 1970s and '80s. Even so, you have to go back a quarter century, to the IRA's burst of bombings in 1992, to find a year with as many attacks as this one.
For Britons, who had reason to feel insulated from the recent wave of terror on continental Europe
, this is a deeply troubling development.
This period includes both far-right attacks, such as last year's murder of Jo Cox
, a member of Britain's Parliament, and radical Islamist incidents, such as the bombing of the Manchester Arena.
It comes amid a challenging environment for intelligence and law enforcement officials. According to the UK Home Office, a record 379 people were arrested on terrorism-related offenses in the past 12 months up to June, a 68% rise on the previous year
The number of prisoners held for terrorism has also swelled
by more than one-third to 204, with the vast majority made up of Islamists. Importantly, there have been 19 plots foiled since 2013, six since March -- one a month, on average.
One reason for this upsurge in terrorist activity and arrests is the international environment. As early as June 2014, half of the casework of MI5, Britain's domestic intelligence agency, involved Britons who had traveled to Syria to fight alongside ISIS and other radical groups.
More recent data
suggest that somewhere between one-half and three-quarters of the 850 Britons who have made that trip have since left the battlefield
. This would still leave more than 200 fighters yet to return. As ISIS loses its grip of cities in Iraq and Syria and its so-called caliphate crumbles, the flow will increase across Europe.
The burden on border control, surveillance, interrogation and prosecution will continue to grow. MI5 -- with a staff of 4,000 or so -- has more than 3,000 "subjects of interest."
Of these, only a small minority, 500 or so, are under active investigation, and an even smaller number under active surveillance.
It would be wildly expensive, incredibly demanding and politically difficult to maintain round-the-clock surveillance on several thousand suspects. It would take at least two dozen people per suspect, which adds up to more than 72,000 personnel -- almost as many as the number of soldiers in the entire British army. In fact, no modern democracy could tolerate such bloated and intrusive security forces.
However, even if this were possible, we should remember that all of this year's attackers -- in Manchester and on Westminster and London bridges -- might not have been designated for surveillance anyway, as they were either in a larger, 20,000-plus category of past subjects of interest, and in some cases entirely unknown to authorities.
If constantly monitoring 3,000 people is impossible, doing so to 20,000 is doubly so
. Mass surveillance is therefore both implausible on democratic grounds and of limited use in practice.
In addition, this year's terror events have often involved rudimentary and unsophisticated means of attack, such as cars, vans and -- in the case of Friday's attempt -- what appears to be a crude explosive device.
Britain benefits from tight gun control laws and -- thanks to its status as an island -- stronger control than other European states over the cross-border flow of firearms. But ISIS has exhorted its followers to lower-tech plots.
Also, in at least three cases, the attacker operated alone. These factors together make it hard for intelligence agencies to detect plotting, because there are fewer points of vulnerability, such as contact with a trained bomb-maker or large numbers of fellow plotters.
The challenge for Britain's police officers and spies is in prioritizing between a large number of threats, accepting that some will always slip through the cracks.
It is also to address the broader, ideological environment in which radicalization takes place, and the international context that allows plotters to seek training, contacts and inspiration from abroad.
Contrary to what was claimed by President Donald Trump
hours after the attack at the Parsons Green station, Britain has been "proactive" in both these areas. The annual counterterrorism policing budget rose 10% from 2016 to 2017
, while the intelligence agencies will have received 1,900 additional staff by 2021.
Even so, the government will be looking to do more. As MI5 chief Andrew Parker has noted, this is a "generational challenge" that will remain for years to come.