As the migration of disaffected young men and women to join the fighting in Syria and Iraq intensifies, so do fears among security services worldwide that their radicalized nationals will bring terrorist plots home with them when the battles are over.
More than 15,000 foreign fighters have traveled to the Middle Eastern war zones, many to join Al Qaeda-linked rebel groups fighting the Syrian government or the Islamic State extremists who have waged a campaign of terror with beheadings, mass executions, kidnappings and rape.
The traveling militants hail from at least 81 countries, security experts report, based on intelligence sources. Their motivations differ broadly.
Some 800 Chechen militants are said to have joined forces with the most extreme groups, seeking to avenge their Muslim communities' abuse by the Russian army and security forces. Former prisoners of the U.S.-run detention facility at Guantanamo Bay, Cuba, have also given rise to new extremist groups after their release from years of imprisonment without charges or a day in court.
Others come from Britain, the United States, France, Australia and other affluent countries, on what analysts describe as a quest for a role in a cause they see as noble and bigger than themselves.
Analysts say months or years of exposure to the brutality and indiscriminate bloodletting that racks Syria and Islamic State's battle for a transnational caliphate are likely to inure the foreign volunteers to violence and leave them vulnerable to recruitment for acts of terrorism against their home countries.
On Tuesday alone, Japanese officials reported the detention of a 26-year-old man they say planned to travel to Syria that day to join Islamic State, federal agents in Indiana revealed that they had intercepted a 19-year-old who they said was en route to fight with the militants, and British authorities arrested four young men suspected of plotting crimes in support of terrorism.
"There is a domestic security risk as some of these people become acclimatized to extremism,"; said Jytte Klausen, an international affairs professor at Brandeis University and founder of the Western Jihadism Project, a data archive on Islamic radicalism in the West.
In her article in the current issue of Foreign Affairs, "They're Coming: Measuring the Threat from Returning Jihadists," Klausen sums up her research into the consequences of "backflow" -- the return from foreign conflicts of fighters who were trained and radicalized by Al Qaeda and its affiliates. Klausen's project identified 900 people who fought in foreign battles between 1993 and 2012 and found that nearly a third of them, 279, took part in violent terrorist plots on Western soil after their return.
The estimated 3,000 Westerners now in Syria and Iraq are younger than those who left to join battles or get training in Bosnia-Herzegovina, Pakistan and Afghanistan as well as more socially diverse, with "gang members from Europe's ethnic enclaves and dropouts from universities and prestigious private schools joining up in equal measure," Klausen notes.
If past propensity to bring violence back home holds with those who will return after the Syrian civil war, that could subject Western countries to about 600 acts of terrorism, or attempts, within a few years, Klausen said.
Jeremy Shapiro, a former State Department policy planner who is currently a fellow at the Brookings Institution, warns of the risks of "hyping" the dangers of foreign fighters threatening their home countries.
The exodus of Western passport holders to the Syrian conflict is "a much larger flow of people this time than ever seen in the past," Shapiro said. But he points to security services' success in intercepting plots and would-be militants, which have prevented all but one known incident of a returning foreign fighter waging a terrorist attack on Western soil.
That case was the May killing of three people in front of the Jewish museum in Brussels by a French citizen linked to the militant group Forsane Alizza, or Knights of Pride.
"That means European security services have a pretty good handle on who's going to Syria and who's coming back," he said, citing half a dozen plots that were foiled by authorities from London to Australia.
Counter-terrorism experts point out that preventing Westerners from joining the extremist groups in the first place is the best protection against their importing violence when they return. But analysts acknowledge that the allure of going abroad to fight for a cause is ascendant, especially since Islamic State rolled through northern and central Iraq in the spring and seized broad territory, looted banks of hundreds of millions of dollars and captured sophisticated weaponry it's now using to conquer more land for a "caliphate."
"There is very much a sense of adventure, a sense of cause, a sense of the desire to do something bigger than oneself" that draws young men, and increasingly young women, to the fight, Shapiro said.
Going to Syria to defend Muslims against aggression by Bashar Assad's government remains "a marginal phenomenon," he said, with 3,000 from Western countries from among the millions of Muslims and disgruntled immigrants.
"We've seen this for all sorts of causes, in all sorts of times in history. Why did so many Europeans and Americans go to fight in the Spanish civil war?" Shapiro said of the 1930s battle that drew the likes of Ernest Hemingway and young idealists from around the world.
Klausen notes that the brutality is part of the attraction for some, providing them with the thrill of victory, "the experience of being in charge, of taking control.... It's exciting."