Dimon's Jewel

MARY ALTAFFER/AP
Bank One CEO Jamie Dimon addresses an analysts meeting with William B. Harrison Jr. of J.P Morgan Chase in New York

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But Dimon was in the game for more than short-term gains, and he needed a major acquisition for the bank to reach critical mass. When Bank of America beat him in the race to take over FleetBoston Financial in October 2003, he knew he had to move fast or get left behind in the rapid consolidation of the banking industry. J.P. Morgan Chase, which had stumbled after the downturn on Wall Street in 2000 and was looking for a bigger retail presence, as well as an eventual successor to CEO Harrison, turned out to be the ideal match. "Every single one of our businesses is enhanced," Dimon said last week. "Even the ones that were Chevys are now Ferraris."

Certainly there's no guarantee that the merger will succeed. The vast majority of big-bank combinations have failed to deliver on their promise, largely as a result of a clash of cultures and loss of focus, and the new J.P. Morgan Chase will still lack the global reach and insurance breadth of Citigroup (the former might also want to acquire a retail brokerage). But as much as anything else, investors are banking on Dimon's stubborn pride to make sure this one will be different. Weill and Dimon may have patched things up at a personal level indeed, Weill was one of the first outsiders to call his former protege to congratulate him on last week's big news. But Dimon, who took boxing lessons during his 18-month sabbatical before joining Bank One, clearly relishes the opportunity to go head to head in business with his onetime mentor.

After all, as Dimon himself pointed out last summer, "When I left Citi, some people said, 'You took business too personally.' But I can't separate the personal and business that readily." In other words, despite his repeated denials, Jamie Dimon always has something to prove, if only to himself.