While many U.S. officials have warned publicly in the last year about the dangers posed by ISIS, the fact that the U.S. intelligence community lacks a consensus estimate on its size and the true identities of the group’s leadership may explain why President Obama over the weekend said the U.S. was caught off-guard by the ISIS advance into Kurdish territory.

That said, the U.S. intelligence community assesses that ISIS poses a particularly difficult problem. One American official said ISIS had attracted thousands of foreign fighters to Syria and Iraq, some of whom had returned to their home countries and formed terror cells in Europe.

ISIS has also proven that it’s practical and can adapt to changing battlefield conditions. Despite taking Iraq’s second largest city, Mosul, in early June, ISIS was smart enough not to try an assault on Baghdad this summer. ISIS would likely lose that battle, according to U.S. intelligence officials, because the city has more than 5 million Shiite Arabs, who belong to the sect of Islam the Sunnis in ISIS consider apostate. While ISIS has refrained from a military assault on Baghdad, U.S. intelligence officials believe the group has cells in place in the Iraqi capital capable of being activated for a future attack or acts of terrorism.