The recriminations grew increasingly bitter Wednesday, as Peter Mandelson, the European negotiator, accused Ms. Schwab of refusing to budge even when her demands were met, and then going public with her grievances too quickly.

Ms. Schwab hit back by criticizing Mr. Mandelson’s negotiating tactics, and argued that the United States farm lobby had been largely supportive of the Doha agenda, which aimed to give smaller and poorer developing countries greater access to consumers in the United States, Europe and Japan.

During the marathon talks, Mr. Lamy sought to accommodate an array of competing interests among developed and emerging nations. His innovation was to include China, India and Brazil in a seven-member group of countries that brokered a draft text. But the breakdown in those discussions highlighted the limits of even that flexible approach.

For Celso Amorim, the Brazilian foreign minister and trade negotiator, the collapse was a sign that that the nature of W.T.O. negotiations had changed for good.

“In the past, it was an E.U.-U.S. business, or, maximum, the Quad,” Mr. Amorim said, referring to a group comprising the United States, the European Union, Canada and Japan that once shaped the direction of trade talks. “Now it is a more complex trading system. You have to look at developing countries as a force.”

Even as the United States dug in against making fresh concessions, China’s efforts to protect its own interests has tilted the playing field in a profound new way. Growing concerns in China about food security appear to have overridden the country’s previous commitment to free trade, which has given it the world’s second-largest trade surplus after Germany’s in recent years.

Since joining the W.T.O. in November 2001, China has been an outspoken defender of free-trade principles. It has been especially critical of the United States, for example, for invoking so-called safeguard rules to prevent an increase of Chinese textile imports that threatened to put American manufacturers out of business.