The penalties associated with the U.S.’ move include asking the IMF to increase surveillance of China's currency practices. The United States could also deny Chinese companies access to financing from the Overseas Private Investment Corporation and prohibit them from bidding on U.S. government procurement contracts. But those do not kick until after a year under the authorizing statute.

However, the Commerce Department has previously proposed treating undervalued currencies as an illegal trade subsidy. If that becomes final, Treasury's designation could help open the floodgates to a number of trade remedies cases seeking countervailing duties on Chinese goods.

Treasury's action "is more symbolic than anything at this point and aimed at Trump's hard core political base that he will need to shore up now that the economy is broadly decelerating in part due to the trade conflict," said Joe Brusuelas, chief economist at RSM. "Moreover, it marks the start of a process that my end in the Trump administration attempting to devalue the dollar."

That possibility was also on the minds of other economists, given Trump's concern that the strength of the dollar hurts the United States in international trade by making its exports more expensive.

"China will view this as unjustified and, therefore, is likely to retaliate, thereby signaling further tension escalation," said Mohamed A. El-Erian, chief economic adviser at Allianz. "It signals a new step in the weaponization of economic instruments, also raising the probability of both trade and currency wars."

Some feared a replay of the disastrous replay of U.S. actions under the Smoot-Hawley Tariff Act in the 1930s and tit-for-tat responses by other countries that many blame for prolonging the Great Depression.