BERKELEY, Calif. — Three years into the financial crisis, one might think that the world could put Great Depression analogies behind it. But they are back with more force than ever. Now the fear is that currency warfare, leading to tariffs and retaliation, could cause disruptions to the international trading system as serious as those of the 1930s.

There's good reason to worry, for the experience of the 1930s suggests that exchange-rate disputes can be even more dangerous than deep slumps in terms of generating protectionist pressures.

In fact, it was not countries' experiencing the worst economic downturns and the highest unemployment rates that raised tariffs and tightened quotas most dramatically in the 1930s.