On September 15, 2008, Lehman Brothers made history by filing the largest bankruptcy in United States History. They filed for Chapter 11 protection and declared $639 billion in Assets and $619 billion in debts. One of the main reasons Lehman brothers crumbled like it did was due to the U.S. subprime mortgage and real estate market. When these markets began to slow down, they started to depend on short term loans in an effort to raise back the billions they lost. However, they were unable to secure funding which would ultimately be the end of Lehman Brothers as one of the largest investment banks in the United States.
Here is an article from the Yale School of Management outlining in great detail the entire story of Lehman Brothers: