Eric Schmitt and Michael S. Schmidt, New York Times, January 12, 2015

For more than a decade, Western governments have struggled to stem the flow of their citizens traveling to fight in war zones in Muslim countries, increasing surveillance of those who have expressed an interest in joining extremists, creating computer programs to track suspicious travel patterns and taking other measures.

But last week’s commando-style raids in France–carried out by at least one man who traveled to Yemen in 2011 to train alongside the Qaeda affiliate there–were deadly reminders that those measures have done relatively little to reduce the threat. The number of people traveling abroad to fight continues to grow, with about 1,000 militant recruits joining the fight in Syria and Iraq each month, according to recent United States government figures.

Worried that these returning militants could go for years without drawing attention, American and European counterterrorism officials have been scrambling to come up with new ways to stop their residents from traveling abroad to fight–efforts that have taken on greater urgency in light of the killings in France.

New or amended counterterrorism laws have been passed in countries like Albania, Australia, Bosnia, France, Kosovo, Macedonia and Serbia, making it illegal to travel to fight in a foreign conflict, like the ones in Yemen, Syria and Iraq.

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The United States seized on the issue last September and successfully pushed through a legally binding resolution at the United Nations Security Council that would compel all countries to take steps to “prevent and suppress” the flow of their citizens to groups considered to be terrorist organizations.

A particular focus has been on countries like Turkey, whose porous 500-mile border has allowed thousands of militants to cross into the Syrian battlefield and into Iraq. In 2013, Turkey denied entry to 4,000 people who had been on a no-entry list and detained more than 92,000 people on its border.

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Altogether, about 18,000 foreign combatants, including 3,000 Europeans and other Westerners, have traveled to fight in the region since the Syria conflict erupted in 2011, according to American intelligence estimates. More than 500 veterans of the Syria campaign have returned to Europe, according to Richard Barrett, a former British intelligence officer who has researched the figures for the Soufan Group, a security consultancy.

In the United States, where about 150 people have tried or actually gone to fight in Syria, federal law enforcement officials have focused not only on monitoring social media networks more aggressively, but also on educating state and local authorities about ways to identify potential travelers.

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Islamic militants have gone to fight in foreign wars dating to the mujahedeen who traveled to Afghanistan in the 1980s to fight the Soviet Army there.

What is different today, terrorism experts said, is the scale of the flow and the various reasons young Muslim men, and increasingly women and families, are flocking to Syria. Young men in Bosnia and Kosovo are traveling to Syria for financial gain, including recruiting bonuses some groups offer, counterterrorism specialists say. Others from the Middle East and North Africa are attracted more by the ideology and the Islamic State’s self-declared status as a caliphate. Counterterrorism specialists have seen criminal gang members from as far as Sweden seeking adventure and violence in the fight.

Fears that former fighters may carry out attacks in their native countries have been intense since Mehdi Nemmouche, a 29-year-old French Muslim, killed four people at the Brussels Jewish Museum last May after spending a year in Syria. The French authorities knew he had gone to fight in Syria and had been told by German officials that he had returned to Europe.

France, with about five million Muslims, the largest Muslim population in Western Europe, has seen a growing number of calls for citizens to turn to jihad. More than 1,000 French citizens have left or plan to leave France to join the ranks of jihadist groups in Syria or Iraq, according to the French Interior Ministry. Mr. Barrett, the former intelligence officer, said perhaps 180 of them had returned.

“We don’t know when these radical people are going suddenly to become terrorists,” Gerard Araud, France’s ambassador to the United States, said on Sunday.

In Britain, about half of the 600 people who have traveled to Syria to engage in terrorist-related activities have returned home, according to the authorities. “Working with our partners, we have stopped three U.K. terrorist plots in recent months alone,” Andrew Parker, the head of Britain’s domestic intelligence service, MI5, said in a rare public speech last week.

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