Some counterterrorism analysts have identified what they say are more positive trends. The more than 7,000 military strikes against Islamic State in Iraq and Syria have largely contained the group in its core territory, they say, and international efforts to strengthen border security and share information about suspected fighters have largely stopped the Islamic State from expanding at the accelerated pace it did in the summer of 2014. Other indicators also suggest that the Islamic State’s ability to recruit and retain followers may be slowing.

A small but growing number of defectors from the Islamic State are risking reprisals and imprisonment to speak out about their disillusionment with the extremist group, according to a report published this month by the International Center for the Study for Radicalization at King’s College London.

“ISIS no longer has the momentum in its core territory of Syria and Iraq,” said Peter Neumann, director of the center and a professor of security studies at King’s College. “It’s no longer the ever-expanding jihadist utopia that it seemed to be.”

In Britain, more than 750 people have traveled to take part in terrorist-related activity in Syria and Iraq, up from about 500 a year ago. About half of those have returned home, raising fears that they could carry out attacks on British soil. And since September 2014, 34 countries, including the United States, have arrested foreign terrorist fighters or aspirants. The United States has active criminal cases against almost 50 foreign fighter suspects.

The United Nations Counter-Terrorism office has recommended that countries take urgent measures to disrupt travel by would-be fighters. At the moment, only five of 21 high-priority countries surveyed require advance passenger information or passenger name records, making it virtually impossible to flag suspects who might be flying to conflict zones in incremental steps, rather than taking direct flights that would invite scrutiny.