As President Barack Obama considered adding as many as 40,000 U.S. forces to a backsliding war in Afghanistan in 2009, Vice President Joe Biden warned him that the military rationale for doing so was flawed, a new book about Obama's expansion of the conflict says.

The book, Little America: The War Within the War for Afghanistan, also says that in planning the drawdown of troops two years later, the White House intentionally sidelined the CIA. Obama purposely did not read a grim CIA assessment of Afghanistan that found little measurable benefit from the 30,000 ‘surge’ forces Obama eventually approved, the book quotes a U.S. official as saying.

A copy of the book by Washington Post correspondent Rajiv Chandrasekaran was obtained by The Associated Press. It will be released on Tuesday.

A previously undisclosed Biden memo to Obama in November 2009 reflects his view that military commanders were asking Obama to take a leap by adding tens of thousands of forces whose role was poorly defined.

Although Biden's doubts have become well known, the new book details how Biden used a months-long White House review of the war to question the basic premise that the same ‘counterinsurgency’ strategy that had apparently worked in Iraq could be applied to Afghanistan.

"I do not see how anyone who took part in our discussions could emerge without profound questions about the viability of counterinsurgency," Biden wrote to Obama. To work, the counterinsurgency or ‘COIN’ doctrine requires military gains to be paired with advances in government services, a ‘credible’ Afghan government and Afghan security services that can take over, Biden's memo said.

Although the U.S. military could accomplish any technical assignment related to the new strategy, such as sweeping insurgents from a village, "no one can tell you with conviction when, and even if, we can produce the flip sides of COIN," Biden wrote. He supported a buildup of 20,000, half the number requested by then-war commander Gen. Stanley McChrystal.

The memo echoed a secret message to Washington from then-U.S. Ambassador Karl Eikenberry that had called Afghan President Hamid Karzai an unreliable partner for the proposed surge. Eikenberry, a former top Army general who had served in Afghanistan, said more forces would only delay the time when Afghans would take over responsibility for their own security.

The Eikenberry memo was leaked shortly after he sent it, and confirmed by U.S. officials. Biden was presumed to agree with it, but he stayed mum at the time.

Obama's compromise - 30,000 additional forces and a deadline to begin bringing them home - was intended to blunt the momentum of a resurgent Taliban insurgency without committing Obama to an open-ended war.

The classified CIA assessment found that Afghanistan was "trending to stalemate" in mid-2011, just ahead of the long-planned date when Obama would begin bringing the additional forces home.

Although many of Obama's advisers had also concluded that the surge strategy had not worked, a White House official is quoted as saying aides initially rebuffed the CIA analysis because it could undercut Obama's argument for withdrawing forces on schedule.

"We didn't want it," the official said.