The American financial system was shaken to its core on Sunday. Lehman Brothers Holdings Inc. faced the prospect of liquidation, and Merrill Lynch & Co. agreed to be sold to Bank of America Corp.

The U.S. government, which bailed out Fannie Mae and Freddie Mac a week ago and orchestrated the sale of Bear Stearns Cos. to J.P. Morgan Chase & Co. in March, played much tougher with Lehman. It refused to provide a financial backstop to potential buyers.

Without...