HONG KONG — China’s central bank unexpectedly announced on Friday night that it would require commercial banks to set aside a bit more money. It was the second such move by Beijing this month and the clearest sign yet that China means to counter the Federal Reserve’s monetary easing in the United States.

But economists were quick to point out that the Chinese central bank had chosen a policy stance particularly well suited to holding down the value of the renminbi against the dollar, at a time when the Federal Reserve was trying to increase the supply of dollars by buying longer-denominated Treasuries.

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The currency wars have started