"Unfortunately, despite all our progress against ISIL on the battlefield and in the financial realm, our efforts have not reduced the group's terrorism capability and global reach," Brennan told the US Senate Intelligence Committee.
"The resources needed for terrorism are very modest, and the group would have to suffer even heavier losses of territory, manpower, and money for its terrorist capacity to decline significantly."
Brennan called Daesh, also known as IS/Islamic State, "a formidable adversary" and even stated that the number of Daesh fighters now "far exceeds what al-Qaeda had at its height."
This warning came after 16 years of US military campaigns across the Middle East. The US-led coalition of 60 nations has also conducted a joint operation against Daesh in Iraq and Syria.
Brennan's words seem to confirm warnings from many on the left that using military strategies to defeat terrorism only leads to the creating further acts of terrorism.
"It should be eminently clear that we cannot bomb Islamist extremists into submission or disappearance," Institute for Policy Studies fellow Phyllis Bennis said in 2014. "Every bomb recruits more supporters."
But Brennan doesn't seem eager to change tactics. He stressed that the CIA will continue "a long and difficult fight" against the terrorist group, which, according to Brennan, "will now intensify its global terror campaign to maintain its dominance of the global terrorism agenda."
It remains unclear if this new push will be any more effective than previous efforts, or whether terrorism poses as great a threat as politicians lead the public to believe, but Brennan noted that the agency is particularly concerned with so-called Western Daesh fighters, who can return to their countries of origin through a variety of means, "including refugee flows, smuggling routes, and legitimate methods of travel."
The agency is also dealing with the "exceptionally challenging" issue of so-called lone wolves like Omar Mateen, who killed 49 people at a gay nightclub in Orlando, Florida over the weekend after claiming his allegiance to Daesh.
Other than the shooter's own claims, the CIA has not yet confirmed any direct link between Mateen and the terrorist group.
Asked what the CIA was doing to counter the group's ability to inspire terrorism abroad, Brennan reportedly said the agency goes "upstream" to identify the source of the propaganda, and then "appropriate actions" are taken against the individuals in Syria and Iraq.
