Obama announces new, leaner military approach.

The U.S. military will steadily shrink the Army and Marine Corps, reduce forces in Europe and probably make further cuts to the nation’s nuclear arsenal, the Obama administration said Thursday in a preview of how it intends to reshape the armed forces after a decade of war.

The downsizing of the Pentagon, prompted by the country’s dire fiscal problems, means that the military will depend more on coalitions with allies and avoid the large-scale counterinsurgency and nation-building operations that have marked the wars in Iraq and Afghanistan.

Instead, the Pentagon will invest more heavily in Special Operations forces, which have a smaller footprint and require less money than conventional units, as well asdrone aircraft and cybersecurity, defense officials said. The military will also shift its focus to Asia to counter China’s rising influence and North Korea’s unpredictability. Despite the end of the Iraq war, administration officials said they would keep a large presence in the Middle East, where tensions with Iran are worsening.

The strategy review was unveiled by President Obama in a rare visit to the Pentagon, where he was flanked by Defense Secretary Leon E. Panetta, the Joint Chiefs and other officials who sought to project an image of undiminished military power even as they gird for an era of austerity that will necessitate a more restrained use of military force and more modest foreign policy goals.

“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.

Obama and Pentagon leaders said their new military strategy, contained in an eight-page document, will guide wrenching decisions on defense cutbacks. Details will be made public in the next few weeks as the White House finalizes its proposed federal budget for the next fiscal year.

In a deficit-reduction deal reached with Congress in August, the White House agreed to cut projected defense spending over the next 10 years by about $480 billion, or about 8 percent of what the Pentagon had planned on.

Although Pentagon officials have portrayed those cuts as painful, Obama said the defense budget is still expected to increase slightly — at about the rate of inflation — each year for the next decade.

“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,” he said.

What really worries Pentagon leaders, however, is that their fiscal problems might well be about to get worse. Under a separate deal with Congress, an additional $500 billion in defense cutbacks will be triggered unless lawmakers can agree on an alternative plan to trim the deficit by the end of the year.

Gordon Adams, a former national security budget official in the Clinton administration, predicted that lawmakers would find a way to avoid that trigger, which Panetta has called a “doomsday” scenario for the Defense Department.