Just eight months ago, Brazilian Finance Minister Guido Mantega declared a “truce” in competitive currency devaluations. Now, Japanese and Swiss moves to weaken the yen and the franc show reviving tension in foreign-exchange markets as the deteriorating U.S. economy weighs on the dollar.

Japan sold yen today, causing the currency to weaken as much as 4 percent against the dollar after rising 5 percent last month. “Ongoing one-sided moves” would hurt the recovery from a March earthquake, Finance Minister Yoshihiko Noda said. Yesterday, the Swiss National Bank cut interest rates to rein in the franc after a gain of about 36 percent in the past 12 months.

Europe's sovereign debt crisis and the battle between Republican leaders and U.S. President Barack Obama over the budget and borrowing limits drove investors to the perceived safety of yen and francs. The risk of a U.S. return to recession, forcing the Federal Reserve to another round of monetary easing, has exacerbated dollar weakness. The currency's drop last year left all of Asia's 10 biggest economies seeking to influence their own exchange rates to aid exporters and growth.