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President Obama offered a new Pentagon strategy that cuts billions of dollars for defense over the next decade and will "turn the page on a decade of war," but has critics arguing will gut U.S. ability to lead a dangerous world.

In a rare appearance in the Pentagon press briefing room Thursday, where he announced that the military will be reshaped over time, Obama said the reality of the U.S. economy is forcing the Defense Department to look at its strategy in an all-new way, even as he insisted that strategy will determine the force structure, "not the other way around."

"We have to renew our economic strength here at home, which is the foundation of our strength in the world. That includes putting our fiscal house in order," the president said.

Calling the "very serious debt and deficit problem here at home" a national security risk, Defense Secretary Leon Panetta, speaking after the president, said curbing the growth in the defense budget from its expected $487 billion in increases over the next decade doesn't mean choosing between national and fiscal responsibilities.

"The president has made clear, and I've made clear, that the savings that we've been mandated to achieve must be driven by strategy and must be driven by analysis, and must not be driven by numbers alone," he said.

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The U.S. will change its investments, he said, noting that the new structure will be presented to Congress in the president's budget to be presented in the coming weeks.

Among the changes, Panetta announced, would be the size of some branches of the Armed Services.

"The Army and Marines Corps will no longer need to be sized to support the kind of large-scale, long-term military operations that have dominated military priorities and force generations over the past decades," Panetta said, adding that forces will have to become more flexible and adaptable to conflicts around the globe.

Panetta said the U.S. will focus its security more on challenges from the Asia-Pacific and Mideast and it must manage the rising cost of health care for military families even as the Pentagon pledges to uphold its commitment to troops. He said in some cases, investment may increase in special operations forces; in new technologies, like unmanned systems; in space and cyberspace capabilities; and on quick mobilization techniques.

That doesn't mean hollowing out the military, he and the president said. Obama stressed in his remarks that the comprehensive defense review that resulted in the new structure emphasizes counter-terrorism, nuclear deterrence, protecting the U.S. homeland and deterring and defeating aggression by any potential adversary.

"Yes, our military will be leaner, but the world must know -- the United States is going to maintain our military superiority with Armed Forces that are agile, flexible and ready for the full range of contingencies and threats," Obama said.

"Even if we didn't have fewer resources, we would expect to change," added Joint Chiefs of Staff Chairman Martin Dempsey, adding that the "two-war paradigm" left over from the Cold War "has been a bit of an anchor, frankly."

As expected, the plan wasn't well received by some on Capitol Hill, including the House Armed Services Committee chairman who described the new approach as more of the Obama administration's strategy to "lead from behind."

"The president has packaged our retreat from the world in the guise of a new strategy to mask his divestment of our military and national defense," said Chairman Buck McKeon, R-Calif.

"This strategy ensures American decline in exchange for more failed domestic programs," McKeon added. "In order to justify massive cuts to our military, he has revoked the guarantee that America will support our allies, defend our interests and defy our opponents. The president must understand that the world has always had, and will always have a leader. As America steps back, someone else will step forward."

"The president comparing our defense spending to the defense spending of other countries is certainly in line with his thinking that America is just like every other country. In terms of our freedoms and our attractiveness as a target, I'm absolutely confident that our enemies do not view us as just another country," said Sen. Roy Blunt, R-Mo.

Obama rejected assertions that the U.S. will be forced into retreat. He added that the budget will still be growing, even if more slowly.

"We have global responsibilities that demand our leadership. In fact, the defense budget will still be larger than it was toward the end of the Bush administration. And I firmly believe, and I think the American people understand, that we can keep our military strong -- and our nation secure --with a defense budget that continues to be larger than roughly the next 10 countries combined."

Panetta said there will be trade-offs, and they will be tough for some, but some of it has to be done to avoid "sequestration," the automatic cuts scheduled for 2013 that resulted after "super committee" lawmakers couldn't agree on a plan to reduce the deficit over the next 10 years by $1.2 trillion.

That's not something that we intend to do," he said of the automatic cuts.