WASHINGTON, July 4 - The Pentagon's most senior planners are challenging the longstanding strategy that requires the armed forces to be prepared to fight two major wars at a time. Instead, they are weighing whether to shape the military to mount one conventional campaign while devoting more resources to defending American territory and antiterrorism efforts.

The consideration of these profound changes are at the center of the current top-to-bottom review of Pentagon strategy, as ordered by Congress every four years, and will determine the future size of the military as well as the fate of hundreds of billions of dollars in new weapons.

The intense debate reflects a growing recognition that the current burden of maintaining forces in Iraq and Afghanistan, along with the other demands of the global campaign against terrorism, may force a change in the assumptions that have been the foundation of all military planning.

The concern that the concentration of troops and weapons in Iraq and Afghanistan was limiting the Pentagon's ability to deal with other potential armed conflicts was underscored by Gen. Richard B. Myers, chairman of the Joint Chiefs of Staff, in a classified risk assessment to Congress this spring. But the current review is the first by the Pentagon in decades to seriously question the wisdom of the two-war strategy.

The two-war model provides enough people and weapons to mount a major campaign, like the Persian Gulf war of 1991 or the invasion of Iraq in 2003, while maintaining enough reserves to respond in a similar manner elsewhere.

An official designation of a counterterrorism role and a shift to a strategy that focuses on domestic defense would have a huge impact on the size and composition of the military.

In a nutshell, strategies that order the military to be prepared for two wars would argue for more high-technology weapons, in particular warplanes. An emphasis on one war and counterterrorism duties would require lighter, more agile forces - perhaps fewer troops, but more Special Operations units - and a range of other needs, such as intelligence, language and communications specialists.

Civilian and military officials are trying to decide to what degree to acknowledge that operations like the continuing presence in Iraq - not a full-blown conventional war, but a prolonged commitment - may be such a burden that it would not be possible to also fight two full-scale campaigns elsewhere.

In effect, the unusual mission in Iraq, which could last for years, has not just taken the slot for one of the two wars; it has upended the central concept of the two-war model. It is neither a major conventional combat nor a mere peacekeeping operation. It does not require the full array of forces, especially from the Navy and the Air Force, of a conventional war, and it takes far more troops than peacekeeping ordinarily would.

The force of 138,000 troops in Iraq is only 13,000 smaller than it was at the height of the offensive on Baghdad two years ago, yet the administration describes the campaign not as a major conventional war, but as the leading effort in the nation's fight against terrorism.

"The war in Iraq requires a very large ground-force presence," said Loren Thompson, an analyst at the Lexington Institute, a policy research center in Arlington, Va. "War with China or North Korea or Iran, the other countries mentioned in the major review scenarios, would require a much more capable Navy and Air Force."

Mr. Thompson added that "what we need for conventional victory is different from what we need for fighting insurgents, and fighting insurgents has relatively little connection to stopping the spread of nuclear weapons. We can't afford it all."

The Pentagon's sweeping study, called the Quadrennial Defense Review, is not due to be completed until early next year, when it will be submitted to Congress with the administration's annual budget request. Yet debate over the review cannot ignore the mounting costs of the war in Iraq, approximately $5 billion a month.

A description of the major issues discussed in the classified review was gathered from interviews with more than a half-dozen civilian officials and military officers from across the armed services who are directly involved in the process.

The current military strategy is known by a numerical label, 1-4-2-1, with the first number representing the defense of American territory. That is followed by numbers representing the ability to deter hostilities in four critical areas of the world, and to swiftly defeat two adversaries in near-simultaneous major combat operations The final number stands for a requirement that the military retain the capability, at the same time, to decisively defeat one of those two adversaries, which would include capturing a capital and toppling a government.

"We have 1-4-2-1 now, and we are going to look at that," said Ryan Henry, who serves as principal deputy under secretary of defense for policy.

Asked where the military's heavy commitment to the fight against terrorism fits into the current strategy formula, Mr. Henry said, "It wasn't there when they came up with 1-4-2-1." If a new strategy emerges from the review, he said, it might be "something that doesn't have any numbers at all."

Several officials involved in the review characterized the debate as "an effort to create a construct that will bring a better balance" among domestic defense, the antiterrorism campaign and conventional military requirements.

After years of saying American forces were sufficient for a two-war strategy, "we've come to the realization that we're not," said another Defense Department official involved in the deliberations, who was granted anonymity because he could not otherwise discuss the talks, which are classified. "It's coming to grips with reality."

Senior leaders are trying to develop strategies that will do a better job of addressing the requirements of antiterrorism and domestic defense, while acknowledging that future American wars will most likely be irregular - against urban guerrillas and insurgents - rather than conventional.

Tentative proposals by midlevel staff members on holding a summer summit on the review have been shelved, and the debate is now driven by weekly meetings that officials say have brought new discipline to a sprawling process.

Under Gordon R. England, nominated to succeed Paul D. Wolfowitz as deputy defense secretary, more than 150 questions that the review should address have been sorted into 36 major themes. They include such things as balancing reserve and active-duty forces; the role of other agencies in domestic security; combat medicine; the ability of foreign coastal powers to keep American forces at a distance; and the ability to attract people with important skills, such as a knowledge of the Arabic language.

The review is analyzing in detail what would happen if the United States had to fight China, North Korea or Iran.

In preparing for the review's presentation to Defense Secretary Donald H. Rumsfeld, the highest-level decisions are made at round-table discussions held about three times a month and managed by Mr. England and Gen. Peter Pace, vice chairman of the Joint Chiefs of Staff and the nominee to succeed General Myers as the chairman. Although no draft of the review has been presented to Mr. Rumsfeld, he already has, in broad terms, endorsed efforts that would transform the military into a lighter, more mobile force.

General Pace declined through a spokeswoman on Friday to discuss the review.

"Whether anybody believed we could actually fight two wars at once is open to debate," one senior military officer said. "But having it in the strategy raised enough uncertainty in the minds of our opponents that it served as a deterrent. Do we want to lose that? We don't want to give any adversary the confidence that they could take advantage of us while we're engaged in one major combat operation."

New York Times