On paper, the strategy is irresistible: snap up a brokerage house, a  mortgage company, or an asset manager and prepare to reap bountiful fee   income while firming up ties to customers. So hoping, one bank after   another this year has announced ambitious acquisitions of nonbank financial   companies.       
These deals, however, are notoriously difficult to execute. In addition  to contending with the normal operational challenges of mergers, banks   buying nonbanks may have to bridge enormous cultural gulfs.   
  
Two deals from the early 1990s serve as cautionary tales. NationsBank  Corp. pushed into the derivatives world in 1993 by buying Chicago Research   and Trading Group, and Chase Manhattan Corp. extended its reach in the home   loan market by buying American Residential Mortgage in 1994.     
But, as the following stories illustrate, both acquirers have  encountered unexpected problems, from staff defections to rocky markets.   Read on.