Can the United States provide the necessary cash, even in the form of private-sector money? The answer is yes — so long as foreign countries are willing to lend it to the United States. For the fact is that America is not only the world's biggest economy. It is also the world's biggest borrower. Its muscular military power is underwritten by foreign capital.

This is an unusual circumstance. In the prime of the European empires, when the British ran much of the Middle East, the dominant power was supposed to be a creditor, not a debtor, investing large chunks of its own savings in the economic development of its colonies. Hegemony also meant hegemoney. Britain, the world's banker before 1914, never had to worry about a run on the pound during its imperial heyday.

But today, as America overthrows "rogue regimes," first in Afghanistan and now in Iraq, it is the world's biggest debtor. This could make for a fragile Pax Americana if foreign investors decide to reduce their stakes in the American economy, possibly trading their dollars for the increasingly vigorous euro.