Could Woodward's Source be ...?
One theme of the book, as so far described, is that Obama wanted to withdraw from Afghanistan and reluctantly agreed to send more troops instead only due to pressure from his military advisors. If things continue to go badly in Afghanistan, that story could be very useful for the President, since it makes it look as though he was the farsighted one in the administration and the mistake of going in instead of out wasn't really his. If, on the other hand, things go well, Obama can take credit for the decisions he made and not worry about being blamed for the decisions he didn't make but, if Woodward is to be believed, wanted to.
Think of it as a win/win strategy.