Securing an extremist state in the Middle East would provide “footholds to attack every apostate government in sight,” a U.S. intelligence official said.

It would also legitimize the struggle of Islamist extremists, in their eyes, since before the September 2001 attacks, U.S. officials said. That would aid in recruitment and give the group further momentum and potency.

The growth of the group also presents the challenge for others of dealing with significant humanitarian consequences. Islamic State militants have said they seek to exterminate any groups that aren’t of like mind, and acted on that by laying siege to thousands of people of the Yazidi religious sect in northern Iraq this month and killing some of them.

Establishing a state would provide a foundation for launching more potentially genocidal operations, something that could bring pressure on the U.S. for more interventions to prevent humanitarian disaster.