LATE one night in September 2008, as the financial world trembled, Josef Ackermann received an urgent call from Berlin.

On the line was Angela Merkel, the German chancellor. She needed his help — now…

Mr. Ackermann, 63, emerged from the panic of 2008 as the most powerful banker in Europe and, depending on whom you ask, possibly the most dangerous one, too. As the chief executive of Europe’s largest bank and a symbol of German financial might, he is at the center of more concentric circles of power than any other banker on the Continent.