Last fall, Spain’s Banco Santander Central Hispano announced that it would pay

.4 billion for a 20% stake in Philadelphia-based Sovereign Bank. The investment gives Santander the right to negotiate a complete takeover of Sovereign after two years. Perhaps some observers regarded the deal as just another acquisition in the wave of consolidations and mergers that has transformed the U.S. retail banking market in recent years. Mauro F. Guillén knew it was much more.

Guillén, professor of international management at Wharton, has watched with great interest the strategic moves of 150-year-old Grupo Santander since it began its current expansion abroad in the early 1980s. His fascination grew in the summer of 2004 when Santander announced the acquisition of Abbey National, a British bank with 18 million customers.…