Sept. 19 (UPI) -- Friday's terrorist bombing in a crowded London subway station - which injured at least 30 passengers but caused no deaths - was the latest in a string of terrorist attacks in Western Europe in recent years.
In mid-August, attacks in Barcelona and the nearby city of Cambrils killed 16 people and injured more than 130 just a few days before an attacker with a knife in Finland killed two and wounded eight others. Earlier assaults this year in London and Manchester, England, left dozens dead and hundreds more wounded.
Since 2015, there has been a sharp increase in both the number of attacks and deaths caused by terrorism in Europe. As someone who studies European security issues, I see three key factors contributing to this development: Europe's large and often poorly integrated Muslim population, proximity to unstable regions like the Middle East and North Africa, and terrorists' new focus on highly vulnerable "soft" targets.
While terrorism in Europe today is commonly associated with Islamic extremism, since the end of World War II Europe has experienced different waves of terrorist violence. In the 1970s and 1980s, the biggest terrorist threat was secular Marxist groups such as the Red Brigades in Italy and Germany's Red Army Faction as well as separatist groups such as Spain's ETA and Northern Ireland's Irish Republican Army.
But since the Sept. 11, 2001, terrorist attacks in the United States, radical Islam has become the greatest terrorist threat facing Europe.
In many Western European countries today, Muslims make up between 5 and 10 percent of the population. Many second- and third-generation European Muslims have struggled more than their parents or grandparents to assimilate, in part due to unemployment and xenophobia.
Their often poor integration creates a large pool of disaffected young people vulnerable to radicalization and extremist violence, though of course only a small number of them turn to terrorism.
In contrast, the United States has a much lower proportion of Muslim residents - about 3.3 million people, or 1 percent of the population - and they tend to be well-integrated into American society, with educational attainment, household income and employment levels comparable to those of the general public.
According to a recent U.S. government study, over a period of more than 15 years, from Sept. 12, 2001, to the end of 2016, radical Islamic extremists killed 119 people in the United States in 23 separate attacks - fewer than the number who died in coordinated attacks in and around Paris on the night of Nov. 13, 2015 (an attack that was, for the record, perpetrated by primarily French and Belgian nationals).
The short road to IS
Geography also works against Europe. For jihadis fleeing the battlefields of Iraq or Syria, Europe is simply easier to reach than the United States or other far-flung Western countries such as Canada or Australia - and vice versa.
As many as 5,000 Europeans left to wage jihad in Iraq and Syria, far more than the number of Americans who joined IS and other terrorist groups. European security services estimate that as many as 30 percent of those fighters have returned home. Many pose little security threat, but there are likely dozens who are actively plotting attacks in Europe.
And thanks to the Schengen Agreement, which dismantled internal border controls within the European Union, terrorists can slip in and out of EU countries with ease - though not necessarily the U.K., which does not participate. Counterterrorism cooperation among European countries has advanced in recent years, but Europe remains a patchwork of security and intelligence agencies.
Even so, Europe still sees far fewer terrorism-related incidents than Afghanistan, Iraq, Nigeria and other parts of the world. In 2016 Western Europe accounted for less than 2 percent of global total terrorist attacks and 1 percent of deaths worldwide.
Nor is this even the worst period of terrorism in modern European history. In 1972, the bloodiest year of Northern Ireland troubles, terrorist attacks killed 400 people in Western Europe, and the region accounted for more than 70 percent of terrorist attacks worldwide.
Still, the number and lethality of attacks in Europe has indeed been rising sharply in recent years, partially as a result of terrorists' changing tactics.
While al-Qaida preferred complex, well-planned assaults like the 9/11 World Trade Center attacks and plans to take down airliners over the Atlantic Ocean, IS has demonstrated a penchant for indiscriminate violence.
Abu Muhammad al-Adnani, a senior IS strategist killed last year, called on Muslims to murder Europeans by any means necessary: "Smash his head with a rock, or slaughter him with a knife, or run him over with your car." He was especially keen on killing "the spiteful and filthy French."
Either through incompetence or sheer good fortune, the crude, homemade bomb that shook London last week failed to detonate properly. The New York Times reported that witnesses on the train "described a tremor, a wave of heat and then a barrage of flames that quickly dissipated."
But elsewhere IS's modus operandi of employing unsophisticated means to spread mayhem has proven deadly. When a van mowed down pedestrians in Barcelona in August 2017, for example, it was the sixth incident of vehicular terror in Europe since last year, following similar attacks in Nice, Berlin, Stockholm and London.
European cities have added new physical barriers to guard against such crude offenses, but police and security services can do little to completely eliminate similar attacks.
Can Europe stay safe?
Security services and law enforcement in Europe are certainly getting better at identifying, tracking and arresting potential terrorists and preventing attacks, though.
Since June 2013, security services in the U.K. have foiled 13 terrorist attacks. And in the dozen-plus years between the Madrid train attack in March 2004 and last month's attacks in and around Barcelona, Spanish authorities stopped a number of potential Islamist terrorist attacks.
Still, Europe will be vulnerable to terrorism for some time. It is virtually impossible to stop someone who is committed to killing civilians using everyday items such as cars, kitchen knives or bombs built with easy-to-find materials like hydrogen peroxide and acetone.
The bigger question is what impact ongoing terrorism attacks will have on European politics and societies. In seeking to reduce the frequency and lethality of inevitable future terrorist attacks, Europe's democratic societies are confronting hard choices.
As in the United States after Sept. 11, Europe is hotly debating the scope of governmental powers, how to guard public spaces without surrendering freedom of movement and ways to better integrate Muslim members of society.
Many Brits seem to accept indiscriminate, jihadist-inspired violence as the "new normal." As one resident of Parsons Green, the site of last week's subway attack, said to The New York Times, "We get attacked, and then we carry on, waiting in anticipation for the next one."