Of course, many countries have manipulated their currencies before  the United States reached a political accord with Japan in the Reagan administration to do exactly that, in an effort to reduce a yawning trade deficit. And for decades, despite global rules prohibiting the practice, countries have sought to help their industries by providing subsidies to companies, as Europe did for years with Airbus, its competitor to Boeing.

But many around the world fear getting trampled as the United States and the Chinese battle each other. Japan intervened in the currency markets recently for the first time in six years, after accusing China of driving the yen up to a 15-year high, in part by buying Japanese debt. But it was a short-term move, many Japanese experts fear. “Japan is in a sense losing out in this competitive devaluation war” through inaction, said Kazuo Ueda, a professor of finance at the University of Tokyo, and a former member of the policy board of Japan’s central bank.

Brazil took similar action and vowed last week to take whatever action it needs to prevent its currency from appreciating. Its finance minister, Guido Mantega, said in an interview that the actions by developed countries, including the United States, to keep interest rates at record lows, one way of devaluing a currency, was a “strategy from the past” that was threatening the economy of Brazil and other “dynamic” emerging markets.

“This is a kind of desperate action taken by countries to try to activate their economies,” Mr. Mantega said. “Since they have not been able to activate their own internal markets, the way out becomes exports. So developed countries work on devaluing their currency in order to become competitive in the few dynamic markets in the world.” Most Western governments, and many economists, place the blame for currency frictions on China, which has refused to let the renminbi trade at anywhere near its real value. Moreover, China has subsidized its exports with artificially low interest rates that shift money from consumers’ bank deposits into cheap loans to businesses.

These tactics are nothing new, especially among the emerging economies of Asia. But experts say the sheer size of the Chinese economy means that its currency policies have global effects.

Not surprisingly, the Chinese see the problem differently. The Chinese press is filled with articles arguing that Americans do not appreciate China’s efforts on their behalf. While other nations’ currencies devalued against the dollar in the 2008 financial crisis, some economists note, the renminbi did not. And while Chinese exports may be artificially cheap, the effect has been to give American shoppers bargains at the expense of Chinese consumers.

“Nobody thinks about that,” Shen Minggao, the chief China economist for Citibank, said in a telephone interview from Hong Kong. “Should China think about the welfare of Chinese consumers, not U.S. consumers?”