Ken Lewis, Bank of America chief executive, has spent months trying to convince investors his bank can expand without big acquisitions, which have long been its strength. But in LaSalle, he confronted a deal he apparently could not refuse.

LaSalle, which BofA proposes to buy from ABN Amro for $21bn, would immediately push BofA to the top of the lucrative Chicago market, one of the few US cities where it has little penetration.