U.S. Presidential candidate Bernie Sanders has criticized banks time and time again, asserting that they are "too big to fail" and that he will take it upon himself to break them up. Let's step back and take a look at how these banks actually got to the point they are at today - here are the 6 biggest banks in the United States by total assets:

1. JPMorgan Chase & Co

The largest bank in the U.S. is the NYC-based JPMorgan Chase & Co (JPM  ). Formed in 2000 from a merger between Chase Manhattan Corporation and J.P. Morgan & Co., the company is worth over $2.4 trillion. This merger effectively combined four of the largest and oldest banking institutions in New York City at the time: J.P. Morgan, Chase, Chemical and Manufacturers Hanover. The resulting firm went through further mergers and acquisitions with Bank One Corp. in 2004, The Bear Stearns Companies Inc. in 2008, and Washington Mutual's banking operations in 2008 as well. The end product is the JPMorgan Chase & Co. that we know today, combining the core company's investment and commercial banking skills with consumer banking, strengthened capabilities in prime brokerage, cash clearing and global energy trading, and a nation-wide branch network that reaches 42% of the U.S. population.

2. Bank of America

Coming in at number 2 is Bank of America (BAC  ), with total assets worth nearly $2.2 trillion. Originally established as the Bank of Italy in San Francisco, subsequent expansion in California and throughout the rest of the nation led to its consolidation with other banking institutions and renaming to Bank of America. Bad hedge fund investments eventually led it to be bought by NationsBank; however, the newly formed bank decided to take on the more well-known name of Bank of America. As a result, the new corporation was headquartered in Charlotte, where NationsBank was and where it remains today. Consumer banking makes up the largest division in BoA, but it also offers services in corporate banking, investment management, and international operations.

3. Wells Fargo 

Next up is Wells Fargo (WFC  ), with assets valued above $1.8 trillion. Since it was founded in 1852, the company has gone through a steady timeline of mergers and acquisitions, including a notably controversial 2008 deal with Wachovia that caused some conflict with Citigroup. In its present form, Wells Fargo is the result of a merger between Wells Fargo & Co. and Norwest Corporation, as well as the Wachovia acquisition. In turn, it has transferred its headquarters to San Francisco. Wells Fargo remains a big player in home mortgaging servicing, deposits, and debit cards, and is consistently recognized for its environmentally beneficial business practices.

4. Citigroup

Rounding out the so-called "Big Four" is Citigroup (C  ). This firm was formed as the result of a 1998 merger between Citicorp, originally chartered by New York State, and Travelers Group. The resulting Citigroup made several more acquisitions before running into trouble during the 2008 subprime mortgage crisis. After a massive stimulus package from the US government, Citigroup stabilized and reorganized into two operating units: Citicorp for retail and institutional clients, and Citi Holdings for brokerage and asset management. After a return to profitability, the company's total assets today are worth $1.8 trillion. Recently, however, Citigroup has seen poor performance, with quarterly profits falling as much as 27% from last year.

5. Goldman Sachs

At number 5 is Goldman Sachs Group (GS  ) with $878 billion in assets. Founded in 1869 in NYC, the company had its IPO in 1999 and has since acquired a multitude of companies that have enhanced its capabilities in investment banking and trading. During the 2008 crash, Goldman managed to profit by short-selling securities backed by subprime mortgages. Afterwards, it transitioned to a traditional bank holding company, and investment banking, lending, management, and institutional client services have been the major functions of Goldman Sachs today.

6. Morgan Stanley

Finally, we end the list on Morgan Stanley (MS  ), which is valued at just over $800 billion. Headquartered in NYC, the company was formed by Henry S. Morgan and Harold Stanley, as well as other J.P. Morgan & Co. partners, in response to the Glass-Steagall Act which required the splitting of commercial and investment banking services. Its main services today include global wealth management, institutional securities, and investment management.