In a discussion with CNN's Jim Sciutto at the U.S. Naval War College, Hagel said the department's mission is to "degrade and destroy" ISIL's capability to threaten U.S. and allied interests around the world.
"We're doing that, as the president said -- not just militarily, because that is but one component," Hagel said. "The president has been very clear on that point."
The defense secretary emphasized that "a stable, new, inclusive government in Iraq, which we're hopeful will be in place next week," also is important. The people of Iraq and the Middle East ultimately will determine their future, Hagel said, and the United States can support them.
Other countries recognize threat
"It's also bringing a group with us of like-minded countries that appreciate the threat that ISIL represents to all of us -- I think you know many of the countries: France, Great Britain, Canada, Australia, Albania [and] others -- to bring that coalition with us," Hagel said.
The defense secretary said the president also has made clear that he wants Congress involved with him, because the effort requires authorizations, airstrikes and funding.
"We've been consulting with the Congress, so it's all of those components, but the mission is very clear," Hagel said. "We're providing the president with those options to degrade and destroy ISIL's capability."
Hagel said ISIL is a threat to the United States and its interests, and allies such as the United Kingdom also have made this point.
"Obviously, Prime Minister [David] Cameron of Great Britain made that pretty clear a couple of days ago," he said. The president, the U.S. attorney general, secretary of homeland security and the director of national intelligence also agree, Hagel said, that ISIL poses "very real threats."
"If they weren't real threats, then the president wouldn't be giving us the mission to go out and degrade and destroy the capabilities [of ISIL]," Hagel said.
Part of that threat, Hagel said, is that more than 100 U.S. citizens who have U.S. passports are fighting in the Middle East with ISIL forces. "There may be more -- we don't know," he said. "We can't take a chance on saying, 'Well, let's technically define this. Is it a real threat today or tomorrow? Or is it going to be in six months?'"
Hagel said ISIL embodies a dangerous ideology of brutality.
"So my job, as secretary of defense, is not to second-guess what may be or what's going to be," the secretary said. "We've got to do everything we can to protect our country [and] our interests at the command of our commander in chief as to what he needs in order to do his job."
The defense secretary described ISIL as a "dangerous a group of people beyond just a group of terrorists."
ISIL controls half of Iraq and Syria today, he added, and must be taken seriously.
In addition to the United States supporting the Iraqi people as they form a new and inclusive government, Hagel said, the president is working to bring together a group of countries to join in the effort. Secretary of State John F. Kerry will be doing this right after the NATO conference, he said, noting he and the commander of U.S. Central Command also will be involved in "bringing a group together that ... can help support forces in Iraq, Syria and the Middle East who respect freedom and dignity and the choices that people will make."
Sense of urgency
"We've got to bring a coalition together," Hagel said, "and do the other things ... that we are doing with a sense of urgency."
Hagel expressed his hope that Congress would approve the president's request for $500 million in funding to support the Syrian moderate opposition.
"This is part of the Counterterrorism Partnership Fund that the president has put forward," Hagel said. "Congress has not acted on that yet. I would hope the Congress would."
The secretary also noted that the president has said there will be no combat action for American troops. "We're not going to do that," he said. "I support that decision. I think it's the right decision."
Hagel said the Defense Department will continue to degrade and destroy ISIL capability. "We will do everything possible that we can do to destroy their capacity to inflict harm on our people and Western values and our interests," he added.
Copyright 2016 Nexstar Broadcasting, Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.