“Between debts and pensions, everyone should realize that this can’t go on forever,” said Kenneth S. Rogoff, an economics professor at Harvard University in Massachusetts and a former chief economist of the International Monetary Fund. “‘We’ll be lucky if it can go on another five to 10 more years.”’

Until recently, debt was hardly a dirty word, especially in Western countries that borrowed to finance economic growth. But few of them managed to shrink their debt when times were good, and instead promised richer pensions and welfare benefits.

In the case of the United States, the surpluses built during President Bill Clinton’s tenure turned into huge deficits during the administration of President George W. Bush.

Now, after bailouts in Greece and Ireland, the unthinkable is becoming more probable. As a result, investors are scrutinizing nearly every country with high debt — with potentially bewildering consequences.

…This month, two ratings agencies warned that the triple-A rating for the United States could be reviewed in a couple of years if the country’s national debt kept growing.

…At the same time, several states, particularly Illinois, are feared to be on the brink of insolvency, possibly requiring a bailout. The burden has mushroomed so quickly that some policy makers are asking Congress to consider the once-unthinkable possibility of allowing states to file for bankruptcy. That may be the only way, advocates of this approach argue, to alleviate overwhelming debts, including huge pension obligations that are siphoning money from education and other state services.

The risk is that the mere talk of such measures could destabilize investors’ faith in United States municipal bond markets.

…Japan is more of a question mark. The Japanese prime minister, Naoto Kan, has warned that the nation faces a financial crisis of Greek proportions if it does not tackle a debt that is expected to rise to 210 percent of the country’s gross domestic product next year.