Journalist Fareed Zakaria is an optimist: He believes in the ability of the United States to adapt. But in his new book, The Post-American World, Zakaria raises a tantalizing argument that the war in Iraq will mark the decline of American power, and that the rise of China, India, Brazil and other countries pose a special challenge to the United States in this century. Zakaria, a columnist and the editor of Newsweek International, talks to Robert Siegel about a possible