The US military has become dangerously overstretched because of the scale of its operations in Iraq and Afghanistan, two reports have warned.
Donald Rumsfeld said the military was capable and battle hardened
One, by former officials in the Clinton administration, said the pressure of repeated deployments was very corrosive and could have long-term effects.
The second, ordered by the Pentagon and yet to be released, reportedly calls the army "stretched to breaking point".
The US defence secretary dismissed the claims as out of date or misdirected.
About 138,000 US troops remain in Iraq, on top of deployments to Afghanistan and Kosovo.
The first study, commissioned by Democratic members of Congress, listed former Defence Secretary William Perry and former Secretary of State Madeleine Albright among its authors.
It said the US military had performed admirably in recent operations but was under "enormous strain".
The studies warn of future problems in recruitment and retention
"This strain, if not soon relieved, will have highly corrosive and potentially long-term effects on the force," it stated.
The report predicted problems recruiting new troops and retaining current ones in the face of repeated overseas tours and shortfalls in vital equipment.
It accused the Bush administration of having failed adequately to assess the size of force and equipment needed in post-invasion Iraq, creating "a real risk of 'breaking the force'."
The report also warned that the lack of a credible strategic reserve "increases the risk that potential adversaries will be tempted to challenge the United States".
The second study, conducted for the Pentagon by military expert Andrew Krepinevich, suggested that the military at its current rate of deployment might not be able to outlast the insurgency in Iraq.
He cited the problems experienced by the army in meeting its recruitment targets last year.
Speaking in Washington, Defence Secretary Donald Rumsfeld rejected the warnings given in both reports, saying: "The force is not broken."
He said the US military was enormously capable and battle-hardened and any report suggesting it was close to breaking point was "just not consistent with the facts".
The BBC's Adam Brookes in Washington says the reports echo the view held by some in Congress and even by some within the armed forces.
They fear that if the Iraq commitment lasts a great deal longer, or if the US is drawn into new conflict, the US armed forces could find it difficult to meet their commitments.
The report came as the British government announced the deployment of thousands of extra troops to Afghanistan as part of a Nato expansion plan in the region.
The troops are likely to be deployed to the south of the country. BBC correspondent Rob Watson says it is unclear whether their role will be to provide support to the Afghan government in the region or to participate in counter-insurgency operations.