Defense Secretary Chuck Hagel will step down, a senior administration official confirmed this morning to Fox News.

Fox News national security correspondent Jennifer Griffin reported on America's Newsroom that the move had been discussed between President Obama and Hagel in recent weeks.

The resignation is being described as mutual by the administration, but Griffin reported that Hagel has been under pressure to leave.

She pointed to recent reports about Hagel standing up to National Security Adviser Susan Rice about the strategy against ISIS.

Fox News senior political analyst Brit Hume discussed with Martha MacCallum what the departure means for the future of the fight against ISIS. 

Hume said he finds it "not credible that he wasn't forced out," pointing out that the Pentagon has talked about the need for more troops to fight ISIS.

"[Obama] believes he was elected to downsize the U.S. military and diminish its footprint around the world and here you had people under his command saying we need to enlarge it to carry out this specific mission of dealing with ISIS," said Hume, noting that Obama's previous two defense secretaries - Robert Gates and Leon Panetta - left with significant disagreements with the president's policies. 

Hume said it's "conceivable" that maybe the president wants a new defense secretary for a stronger effort against ISIS. 

But he said he doubts that is the case based on the president's statements about ISIS so far, adding it's "very odd" for Hagel to be leaving after less than two years on the job.

Watch the full discussion above.