Among the hazards posed by the spiraling chaos in Iraq and Syria is one waiting to occur when it ends: What happens when the thousands of foreign jihadists who have joined the fight return home -- battle-hardened, radicalized and possibly looking for more blood?
Perhaps 8,500 foreign fighters from 74 countries are now in Syria, according to the International Centre for the Study of Radicalisation. Some 60 to 100 of them may be Americans. Hundreds more have come from France, the U.K. and other European countries.
Some might eventually return to the West determined to commit terrorism. They could have some alarming advantages: passports that let them travel freely, familiarity with Western culture and languages, and hospitable home communities, for starters. They may not formally belong to a terrorist group or take orders from a centralized authority, making it harder for law enforcement to predict their behavior.
And as veterans of an increasingly nihilistic religious conflict, they may be violently unstable. In Europe, there's some evidence this nightmare is already unfolding. To stop it from spreading further will require equal parts foresight and forbearance.
The first step is to identify the foreigners already fighting in Syria. This isn't easy. The Federal Bureau of Investigation and the U.S. State Department have teams trying to track them, and the FBI is reportedly monitoring some fighters who have returned from the battlefield.
But aspiring jihadists typically aren't hopping planes to Damascus and going through customs. So tracking them will require a lot of international cooperation. The European Union could help by designating the worst extremist groups in Syria as terrorists and sharing more intelligence among security agencies. Turkey, the entry point for most foreign fighters into Syria, has a special obligation to cooperate.
To some extent, the fighters are making it easier on law enforcement by yakking on social media like giddy teenagers. This suggests one thing not to do: encourage such sites to censor posts from radicals. As distasteful as it may be, those posts provide some of the best leads that law enforcement and intelligence agencies have to work with.
Finally, local police -- who typically understand their own communities better than intelligence agencies do -- have to prepare to counter these threats. Among other things, that means lawfully monitoring cyberspace and looking for signs of radicalization in their neighborhoods. Aspiring terrorists tend to draw support from online communities, and past experience shows that "lone wolf" terrorists tend to blab, online and off, about their extremist beliefs and even their criminal intentions.
The FBI and other federal agencies can help by doing a better job of sharing intelligence with their local counterparts -- a primary recommendation that the House Homeland Security Committee made after the Boston Marathon bombings last year.
Even so, domestic intelligence gathering is a fraught pursuit: It raises intricate privacy concerns, requires immense discretion, and is always susceptible to abuse and overreach -- especially after an attack.
It's prudent to remember that such overreach is itself a primary aim of terrorism. No single attack from an erstwhile Syrian jihadist, however tragic, can do much to threaten the Western way of life. But unwise responses to it very well might. In the long fight against terrorism, that's perhaps the most important lesson: Prevention and resilience need to go hand in hand.
To contact the senior editor responsible for Bloomberg View's editorials: David Shipley at email@example.com.