ISIS, which calls itself Islamic State, burst into global consciousness in June when its fighters seized Mosul, Iraq’s second-largest city, after moving into Iraq from their base in Syria.

The Iraqi Army melted away and Mr. Baghdadi declared a caliphate, or Islamic state, that erased borders and imposed Taliban-like rule over a large territory. Not everyone was surprised by the group’s success.

“These guys know the terrorism business inside and out, and they are the ones who survived aggressive counterterrorism campaigns during the surge,” said one American intelligence official, referring to the increase in American troops in Iraq in 2007. “They didn’t survive by being incompetent.” The official spoke on condition of anonymity because of the delicate nature of the information.