Five years ago, American Express Co. and Discover Financial Services clinched a landmark antitrust victory allowing them to expand their network businesses. Now, the protracted court battles preceding that decision look like the easy part.

October marked the fifth anniversary of the Supreme Court's refusal to hear an appeal by MasterCard International and Visa U.S.A. Inc. in the Department of Justice antitrust case, and the eighth anniversary of the original ruling. That decision freed Visa and MasterCard's member banks to form partnerships with other networks, including Amex and Discover. At first, partnerships proliferated. In 2004, Amex executives even heralded the company's first such link-up, with MBNA Corp., as "transformational."

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