WASHINGTON — President Obama has for the first time put his own stamp on an all-encompassing American military policy by turning from the grinding ground wars that he inherited from the Bush administration and refocusing on what he described as a smaller, more agile force across Asia, the Pacific and the Middle East.

In an unusual appearance at the Pentagon briefing room on Thursday, Mr. Obama outlined a new national defense strategy driven by three realities: the winding down of a decade of war in Iraq and Afghanistan, a fiscal crisis demanding hundreds of billions of dollars in Pentagon budget cuts and a rising threat from China and Iran.

A fourth reality, not mentioned in the briefing room, was Mr. Obama’s re-election campaign and the chorus of Republican presidential candidates who have sought to portray him as decimating the Pentagon budget and being weak in his response to Iran.

Mr. Obama, who spoke surrounded by a tableau of the Joint Chiefs of Staff in dress uniforms and with chests full of medals, underscored the national security successes of his administration — the ending of the Iraq war, the killing of Osama bin Laden and the ouster of Col. Muammar el-Qaddafi of Libya — before declaring that the United States would downsize to a smaller ground force, get rid of “outdated cold war-era systems” and step up investments in intelligence-gathering and cyberwarfare.