Many outside the Fed, and some inside, see the move as a ‘Hail Mary’ pass by Fed Chairman Ben Bernanke. He embraced highly unconventional policies during the financial crisis to ward off a financial-system collapse. But a year and a half later, he confronts an economy hobbled by high unemployment, a gridlocked political system and the threat of a Japan-like period of deflation, or a debilitating fall in consumer prices.

In essence, the Fed now will print money to buy as much as $900 billion in U.S. government bonds through June—an amount roughly equal to the government’s total projected borrowing needs over that period.

In normal times, a Fed spending spree on government bonds would be highly inflationary, because it would flood the economy with money and raise worries about too much government spending. The mere worry of too much inflation in financial markets could drive long-term interest rates higher and cause the Fed’s program to backfire.