The Dollar Index fell by the most since the first quarter of 2011 after the European Central Bank pledged to protect the euro from unraveling and the Federal Reserve committed to reduce unemployment via open-ended debt buying, which may debase the U.S. currency.

Since July 26, when ECB President Mario Draghi said he would do “whatever it takes” to save the euro, the 17-nation currency rose versus 15 of its 16 most-traded counterparts tracked by Bloomberg. Amid the Fed's expansion of monetary stimulus, the Dollar Index lost 2.1 percent in the third quarter. The Bank of Japan, which followed the Fed and the ECB in expanding its balance sheet by 10 trillion yen ($130 billion), is scheduled to announce its next policy decision on Oct. 5.

“The ECB announcement to buy one- to three-year bonds in the periphery” shaped currency markets last quarter, George Davis, chief technical analyst for fixed income and currency strategy in Toronto at Royal Bank of Canada, said in an interview. “It was the opening of a new chapter.”