WASHINGTON >> The Pentagon outlined a plan Thursday for slowing the growth of military spending, including cutting the size of the Army and Marine Corps, retiring older planes and trimming war costs. It drew quick criticism from Republicans, signaling the difficulty of scaling back defense budgets in an election year.

The changes Defense Secretary Leon Panetta described at a news conference are numerous but hardly dramatic. They aim to save money by delaying some big-ticket weapons like a next-generation nuclear-armed submarine, but the basic shape and structure of the military remains the same.

The Army would shrink from a peak of 570,000 to 490,000 within five years, and the Marines would drop by 20,000, to 182,000. Those are considerable declines, but both services will still be slightly larger than on 9/11, before they began a decade of war. Both will keep their footholds abroad, although the Army will decrease its presence in Europe and the Marines plan to increase theirs in Asia.

Panetta said the administration will ask Congress for $525 billion to run the Pentagon in 2013 — $6 billion less than the current budget. War costs, which are not considered part of the base budget, would decline from $115 billion to $88 billion, reflecting the completion of the U.S. withdrawal from Iraq.

The base budget would then increase in each year of the Pentagon’s five-year plan, reaching $567 billion in 2017. A year ago the Pentagon had projected 2017 spending to reach $622 billion. The Pentagon counts those reductions in projected future spending as "defense savings."

When Obama took office in January 2009 the Pentagon’s base budget was $513 billion. In 2001 it was $297 billion.

Under a budget deficit-cutting deal Congress made last summer, the Pentagon is committed to reducing projected spending by $487 billion over the next 10 years. The plan Panetta presented Thursday covers the first five years of that span and would cut a cumulative total of $259 billion in planned spending.

"We believe this is a balanced and complete package," Panetta said.

In a bid to pre-empt election-year Republican criticism, Panetta said the plan begins to shift the Pentagon’s focus from the long wars in Iraq and Afghanistan to future challenges in Asia, the Mideast and in cyberspace. More special operations forces like the Navy SEALs who killed Osama bin Laden will be available around the world, he said, and the Pentagon will stress improvements in cyberdefenses.

Republicans were quick to pounce on the proposed Army and Marine Corps reductions.

"These cuts reflect President Obama’s vision of an America that is weakened, not strengthened, by our men and women in uniform," said Rep. Howard "Buck" McKeon, R-Calif., chairman of the House Armed Services Committee.

McKeon voted for the bill last August that established the requirement for $487 billion in defense savings over five years.

"Taking us back to a pre-9/11 military force structure places our country in grave danger," said Sen. John Cornyn, R-Texas, a member of the Senate Armed Services Committee that will hold hearings on the Pentagon budget plan.

Sen. John McCain, R-Ariz., said the Panetta plan "ignores the lessons of history." He said it provides for a military that is "too small to respond effectively to events that may unfold over the next few years."

The military’s top general, however, defended the administration’s approach. Army Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, said he is convinced that the risks raised by cutting the size of the military are manageable. He said failing to make these changes would have meant even bigger risks.

"This budget is a first step — it’s a down payment — as we transition from an emphasis on today’s wars to preparing for future challenges," he said, adding, "This budget does not lead to a military in decline."

Among other details Panetta disclosed:

–The Air Force would retire some older planes including about two dozen C-5A cargo aircraft and 65 of its oldest C-130 cargo planes.

— The Navy would keep a fleet of 11 aircraft carriers but retire seven cruisers earlier than planned. It also would delay purchase of some other ships, including a new Virginia-class submarine.

–Purchase of F-35 stealth fighter jets, to be fielded by the Air Force, Navy and Marine Corps, would be slowed.

–Current plans for building a new generation of submarines that carry long-range nuclear missiles would be delayed by two years. The current fleet of nuclear-capable bombers and land-based nuclear missiles would be left unchanged.

–Military pay raises will remain on track until 2015, when the pace of increase will be slowed by an undetermined amount.

–Obama will ask Congress to approve a new round of domestic base closures, although the timing of this was left vague and there is little chance that lawmakers would agree to this in a presidential election year.

The defense spending plan is scheduled to be submitted to Congress as part of the administration’s full 2013 budget on Feb. 13.

The defense budget is being reshaped in the midst of a presidential contest in which Obama seeks to portray himself as a forward-looking commander in chief focusing on new security threats. Republicans want to cast him as weak on defense.

Obama has highlighted his national security successes — the killing of bin Laden, the death of other senior al-Qaida leaders and the demise of Libya’s Moammar Gadhafi — to counter Republican criticism. He also has emphasized the completion of the U.S. troop withdrawal from Iraq and the start of a drawdown in Afghanistan as turning points that offer new opportunities to scale back defense spending.

But several congressional Republicans see a political opening in challenging the reductions in projected military spending that the GOP and Obama agreed to last summer as part of a deal to raise the nation’s borrowing authority. They’ve echoed Obama’s potential presidential rivals Mitt Romney, Newt Gingrich and Rick Santorum, who plead for fiscal austerity but contend that sizable cuts would gut the military.

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Associated Press writers Donna Cassata, Lolita C. Baldor and Kimberly Dozier contributed to this report.