Back in December, when President Obama announced he was sending additional troops to Afghanistan, he said something we don't hear very often from the White House.

"We can't simply afford to ignore the price of these wars," Obama said, referring to the actual financial cost of America's engagement in Iraq and Afghanistan.

Michael Mandelbaum, a foreign policy professor at Johns Hopkins University, tells NPR's Audie Cornish that's a rare admission for an American president. And it could signal a new phase of foreign policy for the U.S.

Mandelbaum explores the topic in a new book called The Frugal Superpower: America's Global Leadership in a Cash-Strapped Era. He says America's economic troubles are bound to have an effect on foreign policy, especially once the current crunch gets compounded by massive Social Security and Medicare payments to aging baby boomers.

Mandelbaum says much of our foreign policy over the past 60 years has been driven by one thing: The U.S. was rich.

"For almost all of history, almost every country, when it decided on a foreign policy, had to ask first and foremost: How much it would cost?" In contrast, he says, "We've asked: What's right for the world? What can we do to make the world a better place? What international duties do we have?"

Stepping up to answer those larger questions has made America a kind of de-facto global government, Mandelbaum says, providing governmental services that help keep the world more secure and more prosperous.

Take the dollar, for example, which used to serve as the global currency. In addition, Mandelbaum says, "the United States has been the world's consumer of last resort -- and the United States has played a kind of policing role around the world."

But all of that is coming to an end. Soon, we won't be able to borrow enough money to pay out Social Security and Medicare. "When that happens," he says, "people are going to say, 'Yes, we'd like to have as big a defense budget as possible, but if the choice is between paying out the defense budget and paying out what we are owed in Medicare, we're going to opt for Medicare.'"

Defense Secretary Robert Gates has already begun to talk openly about cuts in defense spending, Mandelbaum points out.

"I hope that it will be possible for the United States to continue many of the important international roles that it plays," Mandelbaum says. But he's not very optimistic.

"I do not think we'll be able to do everything we've done for the past several decades."

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