Former Defense Secretary Chuck Hagel said Sunday the United States should be more focused on defeating ISIS than ousting Syrian President Bashar al-Assad.

NBC Montana reports that Hagel, who was President Barack Obama's second secretary of defense, told CNN's Jake Tapper on "State of the Union" that "ISIS represents the real threat to our country -- the world."

Insisting that Assad must go eventually, though, puts the United States at risk of confusing its allies and adversaries, he said.

"I always thought that we needed to more clearly define our political strategy along with our military strategy," Hagel said.

"There is no military solution to this. We are up against an ideology, we are up against a reality, a set of dynamics we've never seen before -- sophistication of social media, the military prowess, the tactical strategic prowess that ISIS possesses, the funding," Hagel said. "So we should more clearly define, what is our political strategies? What are our priorities? Is Assad the enemy or is ISIS the enemy?"

Hagel said the United States shouldn't become "captive to the differences" and should work with countries it's at odds with -- including Russia and Iran -- against ISIS.

"Let's seize on the common interest. What is the common threat to all of those countries? What's our common interest here? ISIS. And you build around that," Hagel said. "I don't think you're going to find a resolution to Assad until you figure out how you're going to deal with ISIS."

Hagel also discussed a memo he wrote to the White House in October 2014 -- a month before he announced his resignation -- criticizing the U.S. approach to Syria and urging a clearer view of how to handle Assad.

"What's happening here is completely out of control, and there's no prospect for bringing any kind of stability, I think, on the path we're on now, and that was what I was talking about in the memo," Hagel said.