WASHINGTON (CNN) -- The dramatic drop in violence in Iraq is due in large part to a secret program the U.S. military has used to kill terrorists, according to a new book by Pulitzer Prize-winning journalist Bob Woodward.

Bob Woodward's book, "The War Within: Secret White House History 2006-2008," came out Monday.

The program -- which Woodward compares to the World War II era Manhattan Project that developed the atomic bomb -- must remain secret for now or it would "get people killed," Woodward said Monday on CNN's Larry King Live.

"It is a wonderful example of American ingenuity solving a problem in war, as we often have," Woodward said.

In "The War Within: Secret White House History 2006-2008," Woodward disclosed the existence of secret operational capabilities developed by the military to locate, target and kill leaders of al Qaeda in Iraq and other insurgent leaders.

National security adviser Stephen Hadley, in a written statement reacting to Woodward's book, acknowledged the new strategy. Yet he disputed Woodward's conclusion that the "surge" of 30,000 U.S. troops into Iraq was not the primary reason for the decline in violent attacks.

"It was the surge that provided more resources and a security context to support newly developed techniques and operations," Hadley wrote.

Woodward, associate editor of the Washington Post, wrote that along with the surge and the new covert tactics, two other factors helped reduce the violence. Watch Bob Woodward explain the strategy »

One was the decision of militant cleric Muqtada al-Sadr to order a cease-fire by his Mehdi Army. The other was the "Anbar Awakening" movement that saw Sunni tribes aligning with U.S. troops to battle al Qaeda in Iraq.

Woodward told Larry King that while there is a debate over how much credit the new secret operations should get for the drop in violence, he concluded it "accounts for a good portion."

"I would somewhat compare it to the Manhattan Project in World War II," he said "It's a ski slope right down in a matter of months, cutting the violence in half. This isn't going to happen with the bunch of joint security stations or the surge."

The top secret operations, he said, will "some day in history ... be described to people's amazement."

While he would not reveal the details, Woodward said the terrorists who have been targeted were already aware of the capabilities.

"The enemy has a heads up because they've been getting wiped out and a lot of them have been killed," he said. "It's not news to them.

"If you were a member of al Qaeda or the resistance or some extremist militia, you would be wise to get your rear end out of town," Woodward said. "It is very dangerous."

All About Iraq War • Bob Woodward