The report says the military will likely be required to sustain high levels of overseas deployments because of the war on terrorism, and the need for an on-the-ground presence in post-conflict situations, a reference to both Iraq and Afghanistan.
There are neither clear solutions nor any quick fix in sight, it says, adding that the U.S. Army in particular faces a profound challenge.
The author of the report is Lynne Davis, a senior political scientist at the Rand Corporation and a former Under Secretary of State for Arms Control and International Security Affairs.
"The demand for overseas rotations on the part of the army is straining the army," said Lynne Davis. "We currently have some 17 army brigades in Iraq and Afghanistan and those deployments have two important effects. One, the units that are at home have little time to train. They also are not available in too many numbers for other types of contingencies that might arise."
In one of its most telling observations, the report says intense and sustained deployments will place a burden on active members of the military greater than occurred during the Cold War, or U.S. deployments in the Balkans in the 1990s.
U.S. Army regulations call for active duty soldiers to spend two years in the United States for every year in a combat zone. Forces are now stretched so thin, says the report, that many units are able to spend only about one year at home before being deployed again.
The Defense Department has relied heavily on National Guard soldiers in Iraq and Afghanistan. The Rand report says many guardsmen had no expectation when they entered military service that they would be mobilized for as long as they have, adding that lengthy deployments strain military families, and make proper training difficult.
There is no conclusive evidence, says the Rand report, about the long-term impact of prolonged deployment on recruiting or reenlistment rates.
However, it notes that Army National Guard enlistments measured against recruiting goals have dropped by more than 10 percent since 2002, while some monthly recruitment goals this year for the regular army have not been achieved.
Mr. Davis says the report shows that the United States faces some difficult tradeoffs, and must face the reality that large investments will be needed in coming years.
"I don't have a crystal ball," he added. "I don't know how to predict this, but what I do know is my message from the beginning and that is that the army faces some really hard choices and there are no easy or straightforward solutions, and it's not just the army, it is the nation, that maybe if the requirements stay high into the future, that we need to think about some larger commitments in terms of investments for the army."
The findings are not likely to surprise lawmakers who have expressed concern since the United States invaded Afghanistan after the September 2001 terrorist attacks about the ability of the military to handle simultaneous demands around the world.
In recent comments, one lawmaker, Congressman Jack Murtha, expressed concern about the recruitment issue.
"Our military is struggling," he said. "Recruitment numbers tell the story. Every retired General that I talk to says to me, what is the matter with recruitment? Some of the most prominent four star [Generals] ask me when they talk to me, why can't we recruit people for this war?"
The report states that despite steps underway for "military transformation" aimed at creating more streamlined and mobile army brigades, overseas demands will remain high.