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WASHINGTON - President Barack Obama vowed Thursday the United States will remain the world's pre-eminent military power even as the Pentagon scales back spending, shrinks the Army and Marine Corps and pulls back from Europe, giving Asia a higher priority.

In a rare appearance at the Pentagon, Obama spoke as the military released a strategy that summarizes changes planned in how the military will operate in an era of budget cuts that will amount to billions of dollars.

Obama said the U.S. is "turning a page" after having killed al-Qaida leader Osama bin Laden, withdrawn troops from Iraq and begun to wind down the war in Afghanistan. He outlined a vision for the future that would ensure an uncompromised U.S. military strength operating with less money.

Obama announced no new capabilities or defence initiatives. He described a U.S. force that will retain much of its recent focus, with the exception of fighting a large-scale, prolonged conflict like the newly ended Iraq mission or the continuing war in Afghanistan.

His presentation strongly suggests a reduced U.S. military role in Europe, notwithstanding a continuing close relationship with NATO, and says Asia will be a bigger priority. It also emphasizes improving U.S. capabilities in the areas of cyberwarfare, missile defence, intelligence, surveillance and reconnaissance.

"Our military will be leaner, but the world must know the United States is going to maintain our military superiority," Obama said, with Defence Secretary Leon Panetta and the Joint Chiefs of Staff chairman Gen. Martin Dempsey at his side.

Obama said his administration would not repeat the mistakes made after World War II and Vietnam when defence reductions left the military ill-prepared.

"As commander in chief, I will not let that happen again," he said. "Not on my watch."

Both Panetta and Dempsey said they anticipate heavy criticism of their plans, which are meant to guide future defence budgets, including the 2013 spending plan that Obama will submit to Congress in February.

The criticism from Republicans came quickly.

Rep. Howard "Buck" McKeon, chairman of the House Armed Services, issued a statement saying, "This is a lead-from-behind strategy for a left-behind America." He called it a "retreat from the world in the guise of a new strategy."

Dempsey praised the strategy and the work of crafting it, calling it inclusive and comprehensive.

"It's not perfect," the general said. "There will be people who think it goes too far. Others will say it doesn't go nearly far enough. That probably makes it about right. It gives us what we need."

Obama said the strategy overhaul is designed to contend with hundreds of billions of dollars in budget cuts and refocus the United States' national security priorities after a decade dominated by the post.-Sept. 11 wars in Afghanistan and Iraq.

The strategy, devised through a comprehensive review by civilian and military leaders, centred on the military the country needs after the "long wars of the last decade are over," Obama said.

Panetta said that smaller military budgets will mean some trade-offs and that the U.S. will take on "some level of additional but acceptable risk." But Panetta said that at this point in history, in a changing world, the Pentagon would have been forced to make a strategy shift anyway. He says the money crisis merely forced the government's hand.

The president announced that the military will be reshaped over time with an emphasis on countering terrorism, maintaining a nuclear deterrent, protecting the U.S. homeland, and "deterring and defeating aggression by any potential adversary."

Those are not new military missions, and Obama announced no new capabilities or defence initiatives. He described a U.S. force that will retain much of its recent focus, with the exception of fighting a large-scale, prolonged conflict like the newly ended Iraq mission or the ongoing war in Afghanistan.

"As we end today's wars and reshape our armed forces, we will ensure that our military is agile, flexible and ready for the full range of contingencies," the president wrote in a preamble to the new strategy, entitled, "Sustaining U.S. Global Leadership: Priorities for 21st Century Defence."

Obama's decision to announce the strategy himself underscores the political dimension of Washington's debate over defence cuts. The administration says smaller Pentagon budgets are a must but will not come at the cost of sapping the strength of a military in transition, even as it gets smaller.

In a presidential election year, the strategy gives Obama a rhetorical tool to defend his Pentagon budget-cutting choices. Republican contenders for the White House already have criticized him on a wide range of national security issues, including missile defence, Iran and planned reductions in ground forces.

Obama also wants the new strategy to represent a pivotal point in his stewardship of defence policy, which has been burdened throughout his presidency by the wars he inherited and the drag these conflicts have placed on military resources.

The new strategy moves the U.S. further from its longstanding goal of being able to successfully fight two major regional wars -- like the 1991 Gulf War to evict Iraqi forces from Kuwait or a theoretical ground war in Korea -- at the same time.

The document released Thursday made clear that while some current missions of the military will be curtailed, none will be scrapped entirely.

"Wholesale divestment of the capability to conduct any mission would be unwise, based on historical and projected uses of U.S. military forces and our inability to predict the future," the document said.

It said the U.S. will maintain a robust nuclear arsenal but hinted at reductions.

"It is possible that our deterrence goals can be achieved with a smaller nuclear force, which would reduce the number of nuclear weapons in our inventory as well as their role in U.S. national security strategy," the strategy said.

The administration and Congress already are slashing projected defence spending to reflect the closeout of the Iraq war and the drawdown in Afghanistan. The massive $662 billion defence budget planned for next year is $27 billion less than Obama wanted and $43 billion less than Congress gave the Pentagon this year.

A prominent theme of the Pentagon's new strategy is what Panetta has called a renewed commitment to security in the Asia-Pacific region.

The administration is not anticipating military conflict in Asia, but Panetta believes the U.S. got so bogged down in Iraq and Afghanistan after 9-11 that it missed chances to improve its strategic position in other regions.