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The frightful Battle of Paris that many observers expected has been averted — at least for now. In unveiling mammoth plans to modernize and reorganize France's capital and its surrounding suburbs, French President Nicolas Sarkozy set a flexible, all-inclusive tone. That was in stark contrast to his earlier comments (and habitual leadership style), which suggested that the creation of Greater Paris would be done his way or not at all.

But even as he called on various governing authorities, private and public organizations, and ruling conservatives and leftists alike to unite in what he called an "ambitious and difficult" undertaking, Sarkozy left two major questions unanswered, both of which promise to provoke clashes in the future: who will foot the bill, and who will...