NEW YORK (AP) - America's biggest banks have suffered unprecedented losses from the ongoing credit crisis, and that's made some investors question whether the big financial conglomerates should be broken up in order to survive.

Break-up advocates, who for months have been clamoring for Citigroup Inc. (C) to be dismantled, got some validation of their viewpoint this past week. Europe's UBS AG (UBS) - created through the combination of Swiss Bank Corp. and Union Bank of Switzerland in 1997 - on Wednesday laid the groundwork to tear up its business model after another quarter of steep losses.

Though the UBS announcement was expected, it was nonetheless a departure from what executives promised during a wave of big bank deals that began in the late 1990s. The creators of global banks like Citigroup, JPMorgan Chase & Co. (JPM), and HSBC Holdings PLC (HBC) had promised customers and shareholders that a diverse set of businesses would shield them from economic volatility.

But, those models haven't sheltered the banks from the subprime mortgage crisis that turned into a dislocation of the credit markets. Major global banks have taken more than $300 billion in asset write-downs, and organizations like the International Monetary Fund believe that amount could reach $1 trillion. ...