J.P. Morgan Chase is the second-largest bank in the U.S. behind Citigroup, which was the world’s largest before the merger between Japan’s Mitsubishi Tokyo Financial Group and UFJ Holdings was completed in October. Jamie Dimon, Bank One’s former chief executive, who will ascend to J. P. Morgan Chase’s top job by 2006, is ready to take on his former employer, Citigroup. Dimon had been the protégé of Citigroup chairman Sanford Weill, but was ousted in 1998.