By intervening, though, Japan and Switzerland risk criticism that they are inciting what some market players call “currency wars,” where countries compete to devalue their currencies.

Both countries also devalued their currencies last year. South Korea and Brazil intervened in foreign exchange markets earlier this year. And China has long purchased dollar- and yen-denominated assets in an effort to keep its renminbi weak enough to sustain its export economy.

“In a dream world where the Ministry of Finance and Bank of Japan could dictate exchange rates, they certainly won’t mind to see the yen weaken to 85-90 yen against the dollar,” Takuji Okubo, chief economist in Tokyo for Société Générale, wrote in a note to clients. “However, with all the developed economies in the world suffering, trying to grow through a weaker currency is likely to encounter resistance.”

But Japan right now sees itself having little choice.

“The recent rise in the yen in currency markets has been one-sided and unbalanced,” the finance minister, Yoshiko Noda, said on Thursday as he announced the start of the intervention. “If this trend were to continue, it would harm the Japanese economy, even as we do all we can to recover from our natural disasters.”

On Thursday, Japanese authorities delivered a one-two punch. First, the Finance Ministry said it had begun selling yen and buying dollars. Then the Bank of Japan announced that it had further expanded its program to buy government and corporate bonds, a form of monetary easing aimed at increasing liquidity and helping to dilute the value of the yen.