For a quarter-century after World War II, money was based on a loose version of the gold standard. The U.S. dollar was pegged to gold; other currencies were pegged to the dollar; stable prices underpinned the prosperity and soaring trade of the 1950s and '60s.

Then in 1971, Richard Nixon balked at the high interest rates necessary to maintain the dollar's link to gold. For the rest of the decade, inflation ripped. The cure, starting in 1979, involved two recessions in the United States and the Third World debt crisis.

Now we face another potential watershed in the world's system of money. Since the breaking of the gold link, the dollar has become the world's primary measure of value, so much so that bank deposits in Uruguay and bribes paid in Russia are mostly denominated in dollars. But the dollar, like the gold standard before it, is under pressure. Last week even Giselle Bundchen, the world's top supermodel, was reported to be steering clear of greenbacks.

Inflation was the cause of the gold standard's collapse as well as its main consequence. As long as the dollar was convertible, investors could choose between owning one dollar and owning one-35th of an ounce of gold; when inflation eroded the greenback's purchasing power, gold was the more attractive option. Foreigners traded in their dollars until U.S. gold stocks were close to exhaustion. Higher U.S. interest rates could have lured foreigners back into dollars. But Nixon wouldn't tolerate high rates the year before an election.

Today's problem is different. The Fed has kept a lid on inflation, but the dollar's vulnerability is caused by debt — the debt of the federal government and of American households. Year after year, foreigners have provided Americans with the savings that they refuse to generate themselves, and this stream of money has supported the U.S. currency. But if foreigners tire of handing over their savings, the unsupported dollar is almost bound to fall. That is what has happened recently.

Once a currency ceases to act as a store of value, its days as a reserve currency — that is, a currency in which foreigners are happy to hold savings for the long term — may be numbered.

As in 1971, the Federal Reserve could do something. It could keep interest rates high enough to entice investors to hold dollars. But as in 1971, this is not an attractive option. The U.S. economy is reeling under the impact of an oil shock, a housing shock and financial turmoil. Forced to choose between upholding the dollar's role as an international store of value and avoiding domestic recession, the Fed is likely to prioritize recession-avoidance.

Nixon's Treasury secretary, John Connally, told furious Europeans that the dollar was "our currency, but your problem." The same could be said for today's dollar trouble, which is why French President Nicolas Sarkozy said plaintively last week that "the dollar cannot remain someone else's problem."

For the U.S., a falling dollar means pricier imports but also an export boom that could carry the U.S. economy through its housing bust. Yet for France and other countries that use the euro, a weak dollar means a loss of competitiveness — not only against U.S. producers but also against dollar-pegging Asian exporters.

The falling dollar is a head-ache for the dollar-peggers, too. Their problem is the mirror image of the European one: Countries such as China and the Arab Gulf states are already experiencing an export boom that is overheating their economies. As a falling dollar drags down their currencies, this overheating gets worse. Meanwhile, they have accumulated vast piles of dollar assets that are now losing value. Saving on America's behalf turns out to be expensive.

So the world faces a dilemma. The last thing it wants is more dollar weakness, which is why central bankers in East Asia and the petro-states, which control most of the world's official reserves, are not about to dump U.S. bonds and trigger a collapse in the greenback. But the world may also draw the lesson that an alternative global currency needs to be the long-term goal. Households don't like saving in a currency that won't hold its value. Companies don't like building global supply chains based on a unit of account that fluctuates unstably.

Most economists assume that the dollar will continue to act as the global currency because there is no alternative. But one of my colleagues at the Council on Foreign Relations, Benn Steil, has proposed another option — a privately created currency that would confer an inflation-proof claim on gold or a basket of commodities. Steil calls his idea "digital gold," which has a nice back-to-the-future ring.

The more the dollar slides, the less Steil's suggestion sounds like a fantasy from a movie studio.

Sebastian Mallaby is a fellow for International Economics with the Council on Foreign Relations. He wrote this commentary for The Washington Post.