This is an excerpt from Every Nation for Itself: Winners and Losers in a G-Zero World, published this week by Portfolio.

Is the U.S. really in decline? Can China become a superpower? Can Europe rebuild? How fast can the rest rise? These are interesting issues, but today’s world faces a more urgent and important question: While we’re figuring all that out, who will lead? Unfortunately, the answer is no one. In this G-Zero era, no one is driving the bus.

The United States and its European allies can no longer drive the global political and economic agenda. The scramble to produce a coordinated and effective multinational response to the 2008 financial crisis made that clear, but the growing leverage of emerging states like China, India, Brazil, Russia and others was apparent years before U.S. financial institutions began melting down and the Eurozone descended into crisis.

Yes, America remains the most powerful and influential country on Earth – and will for the foreseeable future. Its economy is still the world’s largest. No single nation can compete with its cultural influence, and only America can project military power in every region. But in coming years mounting federal debt and the domestic political attention now focused on this issue will force the architects of U.S. foreign policy to become more sensitive to costs and risks when making potentially expensive strategic choices.

At home, it will be harder for presidents to persuade taxpayers and lawmakers that bolstering the stability of countries like Iraq or Afghanistan is worth a bloody, costly fight. That means decoupling support for a “strong military,” an always popular position, from security guarantees for countries that no longer meet a narrowing definition of vital U.S. interests. Abroad, questions will arise about America’s commitment to the security of particular regions, encouraging local players to test U.S. resolve and to exploit any weakness they think they’ve found. Few want a global policeman, but some will have second thoughts when they realize they lack protection against a neighborhood bully.