These days, any assessment of American foreign policy seems to circle back to whether we can get our act together at home.

This country cannot command respect overseas when its domestic politicians act like irresponsible children. The world looks agog at our paralyzed Congress. A sagging superpower unable to confront the challenges that face it.

Given this sad state of the union, I was drawn to the title of a new book by uber-pundit Thomas L. Friedman and noted political scientist Michael Mandelbaum: “That Used to Be Us: How America Fell Behind in the World It Invented and How We Can Come Back.”

Sadly, the book doesn’t deliver on the last part of the title. Yet it bluntly describes what has sent us into slow but steady decline.

If there’s one word to take away from this book it is adaptation. When the Cold War ended, the United States basked in its “victorious” role as sole superpower. But, say the authors, we failed to grasp that communism’s end dramatically accelerated the process of globalization.

Other countries, notably China, emulated our past open-market success. They adapted to the new world and we didn’t. “We relaxed, underinvested, and lived in the moment just when we needed to study harder, save more, rebuild our infrastructure” and attract more, not less, skilled immigrants. As Friedman and Mandelbaum write: “The failure to understand that we were living in a new world, and to adapt to it was a colossal and costly American mistake.”

The consequences of this miscalculation are stunning. Just a few:

On infrastructure: As China builds modern roads, rail, and airports all over its vast landmass, our roads, bridges, water systems, and electric grids have eroded from lack of investment. In 2009, the American Society of Civil Engineers gave America a D grade on 15 categories of infrastructure.

On education: As global competition demands ever higher educational skills, we are producing a permanent underclass of youths without a high school education. According to Secretary of Education Arne Duncan, “Currently, about one-fourth of ninth graders fail to graduate high school within four years.” Of the major industrialized countries, only Mexico, Spain, Turkey, and New Zealand have higher dropout rates than the United States.

On immigration: As politicians whip up hysteria on this issue, we are discouraging talented immigrants who are “critical to our long-term economic health.” According to the Indian-born U.S. immigration scholar Vivek Wadhwa, 52 percent of Silicon Valley’s tech companies were started by immigrants, who also contributed to 25 percent of U.S. global patents. Immigrants make up 24 percent of science and engineering workers with bachelor’s degrees in this country, and 47 percent of those with doctorates.

Our failure to welcome talented immigrants eats away at our ability to keep our competitive edge. Meantime, our own students score below the international average in comparative tests of math and science.

Of course, our bipartisan run-up of debt over the past decade (destroying the hefty surplus built up under President Bill Clinton) compounds all these problems. The authors decry the political paralysis that blocks any realistic approach to the deficit.

But this book reaffirms my belief that our core problem is much deeper than the political impasse on Capitol Hill.

“The American people still have not fully understood the world we are living in,” Friedman and Mandelbaum write. Most Americans understand something has gone deeply wrong with the country. But they don’t grasp the causes of our decline — or how to reverse it.

Irresponsible politicians offer dangerous bromides such as “Take back our country,” with its racist, anti-immigration tones. They rail against “big government” as if government wasn’t essential to shaping change.

Yet this country has deep structural problems that won’t be resolved by calls for a return to past glories. These problems cannot be addressed until the public is rallied and clearly told why the country is declining. None of our politicians seem equal to the task.

President Obama hasn’t managed to galvanize the nation. Republicans are too busy denouncing Obama, science — and taxes. The tea party is a dangerous distraction. And the media are so fragmented they only add to the cacophony.

Friedman and Mandelbaum have given up on both major parties and call for a third party of the “radical center” that would focus public attention on the real crises we face. That scenario seems unlikely — and insufficient.

Unless a leader arises — from whatever party — who tells Americans the truth and can rally them ’round — the downward slide will continue. “If we were behaving like our parents’ generation we’d get this done,” says Mandelbaum. But we’re not.