The president is particularly concerned about the potential for a nuclear weapon being detonated in a major American city. Woodward reports on a secret war game where officials pretend that terrorists had just blown up a nuclear weapon in Indianapolis and are said to be plotting against Los Angeles next; administration officials appear shaken by the exercise and the lack of preparation for its fallout.
Obama told Woodward that a nuclear weapon in the hands of a terrorist is a "potential game-changer" that he ponders constantly: "When I go down the list of things I have to worry about all the time, that is at the top, because that's one area where you can't afford any mistakes," Obama said.
Much of the book focuses on the drawn-out internal deliberations about Afghanistan policy, including some 40 private conversations between Obama and top aides, and more than 20 closed-door strategy sessions.
"There's this kind of hot-house of action. And one of the things you find is, Obama drives them," Woodward said in the interview. "'I want answers. What about this, what about that?' And it's not exactly a relaxing job for him or for the people who work there."
The debate is rigorous and, at times, personal. National Security Adviser James Jones, a retired Marine general who joined the administration from outside the Obama inner circle, dismisses the president's political aides as "the water bugs" and the "campaign set," Woodward writes.
Gen. David Petraeus, who was among those calling for a larger troop presence, calls senior White House adviser David Axelrod "a complete spin doctor." Axelrod voices mistrust for Hillary Rodham Clinton from the time president-elect Obama told him he was considering choosing her for secretary of state.
And Vice President Joe Biden is quoted as calling Richard Holbrooke, the president's special representative for Afghanistan and Pakistan, "the most egotistical bastard I've ever met."
On just the president's fourth day in office, Petraeus -- who then headed U.S. Central Command, giving him oversight of the wars in both Iraq and Afghanistan -- outlined his recommendations for the Afghanistan conflict.
Petraeus then announced that he was set to move forward with an order for 30,000 new troops, despite the fact the president hadn't signed off on that and had even left the room.
"Rahm Emanuel steps in like a sledgehammer and says, 'General, I know you're doing your job. Thank you. But I didn't hear the president make that decision,'" Woodward told Sawyer.
Watch Diane Sawyer's interview with Bob Woodward tonight on "World News" at 6:30 p.m. ET and more on "Nightline" at 11:35 p.m. ET
The book shows Obama's focus turning increasingly to Pakistan, even as his administration's military efforts continue to be directed at Afghanistan. Woodward reveals the existence -- since confirmed by ABC News -- of a secret, 3,000-member CIA-controlled paramilitary army that's conducting operations in both Afghanistan and Pakistan.
Woodward said the president was told of deep problems in the US relationship with Pakistan in his very first intelligence briefing -- "a cold shower" that came just days after his triumphant victory in the 2008 presidential election.