Such terrorist acts are likely to continue and even intensify, at least initially, analysts say, as the group evolves from a quasi-state with territorial holdings to a shadowy and diffuse network with branches and cells on at least three continents.

Indeed, while the loss of a physical sanctuary would constitute a major blow to the Islamic State — severely limiting, for example, its ability to raise money, train recruits or plan complex terrorist operations — the group’s highly decentralized nature ensures that it will remain dangerous for some time to come, according to current and former U.S. officials and terrorism experts.

“Where al-Qaeda was hierarchical and somewhat controlled, these guys are not. They have all the energy and unpredictability of a populist movement,” said Michael Hayden, the retired Air Force general who headed the CIA from 2006 to 2009.

Islamic State officials, in public statements and in interviews, insist that the group’s “caliphate” project remains viable while also acknowledging that military setbacks have forced a change in strategy.

“While we see our core structure in Iraq and Syria under attack, we have been able to expand and have shifted some of our command, media and wealth structure to different countries,” a longtime Islamic State operative, speaking through an Internet-based audio service, said in an interview.

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