U.S. Secretary of Defense Leon Panetta says it will be “very difficult” to cut more than $450 billion over the next decade from the nation's defense budget, but he says he also sees it as an opportunity to reshape U.S. armed forces for the future.

Panetta told reporters Tuesday that defense spending has nearly doubled in the 10 years of war in Afghanistan and Iraq. He said finding hundreds of billions of dollars in savings will involve “tough trade-offs,” but he called it an opportunity to re-examine military priorities and turn the U.S. armed forces into a smaller and more agile force.

Speaking at the same news conference, Joint Chiefs of Staff Chairman Admiral Mike Mullen said he believes it is “achievable” and “responsible” to make the congressionally mandated spending cuts. But Mullen said he fears that any additional “devastating” cuts would make the military unable to effectively do its job.

The $450 billion figure was part of a compromise agreement earlier this year that requires drastic spending cuts in return for raising the U.S. borrowing limit. That agreement also called for cutting an additional $500 billion from the national security budget if lawmakers cannot agree on a targeted deficit reduction plan.