The debate over economic policy has taken a predictable yet ominous turn: the crisis seems to be easing, and a chorus of critics is already demanding that the Federal Reserve and the Obama administration abandon their rescue efforts. For those who know their history, it’s déjà vu all over again  literally.

For this is the third time in history that a major economy has found itself in a liquidity trap, a situation in which interest-rate cuts, the conventional way to perk up the economy, have reached their limit. When this happens, unconventional measures are the only way to fight recession.

Yet such unconventional measures make the conventionally minded uncomfortable, and they keep pushing for a return to normalcy. In previous liquidity-trap episodes, policy makers gave in to these pressures far too soon, plunging the economy back into crisis. And if the critics have their way, we’ll do the same thing this time.

The first example of policy in a liquidity trap comes from the 1930s. The U.S. economy grew rapidly from 1933 to 1937, helped along by New Deal policies. America, however, remained well short of full employment.