For more than six years now, the United States has in fact been fighting two wars, with more than 170,000 troops now deployed in Iraq and Afghanistan combined. The military has openly acknowledged that the wars have left troops and equipment severely strained and has said that it would be difficult to carry out any kind of significant operation elsewhere.

To some extent, fears have faded that the United States may actually have to fight, say, Russia and North Korea, or China and Iran, at the same time. But if Iraq and Afghanistan were never formidable foes in conventional terms, they have already tied up the American military for a period longer than World War II.

A senior Defense Department official involved in a strategy review now under way said that the Pentagon was absorbing the lesson that the kinds of counterinsurgency campaigns likely to be part of some future wars would require more staying power than in past conflicts, like the first Gulf War in 1991, or the invasions of Grenada in 1983 and Panama in 1989.

In an interview with National Public Radio last week, Defense Secretary Robert M. Gates made it clear that the Pentagon was beginning to reconsider whether the old two-wars assumption "makes any sense in the 21st century" as a guide to planning, budgeting and weapons buying.

The discussion is being prompted by a top-to-bottom strategy review that the Pentagon conducts every four years, as required by Congress and officially called the Quadrennial Defense Review. One question on the table for Pentagon planners is whether there is a way to reshape the armed forces to provide for more flexibility in tackling a wide range of conflicts.

Among other questions are the extent to which planning for conflicts should focus primarily on counterinsurgency wars like those in Iraq and Afghanistan, and what focus remains on well-equipped conventional adversaries like China and Iran, with which U.S. Navy vessels have clashed at sea.

Thomas Donnelly, a defense policy expert with the conservative American Enterprise Institute, said he believed that the Obama administration would be seeking to come up with "a multiwar, multioperation, multifront, walk-and-chew-gum construct."

"We have to do many things simultaneously if our goal is to remain the ultimate guarantor of international security," Mr. Donnelly said. "The hedge against a rising China requires a very different kind of force than fighting an irregular war in Afghanistan or invading Iraq or building partnership capacity in Africa."

But Mr. Donnelly cautioned that the review now under way faced a familiar challenge. "If there has been one consistent thread through all previous defense reviews," he said, "it is that once the review is done, there is an almost immediate gap between reality and force planning. Reality always exceeds force planning."

It already is obvious, a senior Pentagon official said, that the Defense Department will "need to rebalance our strategy and our forces" in a way that reflects lessons from Afghanistan and Iraq.

Michael E. O'Hanlon, a senior fellow with the Brookings Institution, a liberal-centrist policy organization, said that senior Pentagon officials knew that the new review needed to more fully analyze what the rest of the government could bring to national security.

"We have Gates and others saying that other parts of the government are under-resourced" and that the Department of Defense should not be called on to do everything, Mr. O'Hanlon said. "That's a good starting point for this — to ask and at least begin answering where it might be better to have other parts of the government get stronger and do a bigger share, rather than the Department of Defense."

Among the refinements to the two-wars strategy the Pentagon has incorporated in recent years is one known as "win-hold-win" — an assumption that if two wars broke out simultaneously, the more threatening conflict would get the bulk of American forces while the military would have to defend along a second front until reinforcements could arrive to finish the job.

Another formulation envisioned the United States defending its territory, deterring hostility in four critical areas of the world and then defeating two adversaries in major combat operations, but not at exactly the same time.

The most recent strategy of the administration of George W. Bush, completed four years ago, added requirements that the military be equipped to deal with a broad range of missions in addition to war-fighting, including defeating violent extremists, defending U.S. territory, helping countries at strategic crossroads and preventing terrorists and adversaries from obtaining biological, chemical or nuclear weapons.

But Pentagon officials are now asking whether the current reality of Iraq and Afghanistan really fits any of those models.