Leon Panetta, sworn in Friday as the nation’s 23rd secretary of defense, pledged there would be “no hollow force on my watch,” as the Pentagon seeks to rein in costs without undermining military readiness.

Panetta faces an administration request to identify $400 billion in savings in the defense budget over the next 12 years, a target that will require him to make hard choices.

In an opening message to Defense Department personnel, Panetta acknowledged the scope of the fiscal challenges but said he believes in achieving a balance between disciplined spending and doing what is necessary to maintain the strength of U.S. forces.

“While tough budget choices will need to be made, I do not believe in the false choice between fiscal discipline and a strong national defense,” he said, virtually echoing remarks by his predecessor, Robert. M. Gates. “We will all work together to achieve both.”

Gates began squeezing some reductions out of the Pentagon budget before his departure, outlining ways to save $178 billion over the next five years. To identify further savings, the Pentagon has established a broad review process, and it will be up to Panetta, a former director of the Office of Management and Budget, to make the strategically difficult decisions.

While Gates largely backed President Obama’s calls for deeper cuts, he warned that reductions must be executed with an eye toward strategy and could not be carried out simply as a “math exercise.”

In an op-ed in the Wall Street Journal on Friday, former defense secretary Donald H. Rumsfeld warned that it would be a “grievous mistake” for Panetta to accede to Obama’s proposal to carve $400 billion from the defense budget.

“While there are substantial savings to be found in the defense budget, hundreds of billions cannot be cut without impairing our security,” Rumsfeld said. “Mr. Gates has said that he’s already made the ‘easy’ cuts, yet there are serious questions whether some of them . . . leave America ill-prepared for a conventional conflict and erode the strong deterrent necessary to prevent it.”

Rumsfeld’s tenure at the Pentagon under president George W. Bush was marked by big increases in the defense budget and few cuts. Gates described the hundreds of billions of dollars in cuts he made in 2009 as “no-brainers.”

Staff writer Greg Jaffe contributed to this report.