The lenders – both based in Frankfurt and 150 next year – were set up to support German companies trading around Europe and beyond. The German government still views that role as crucial and favoured a merger.
Here's a timeline of key events that show how the banks got here.
How it all started
1870: Deutsche Bank is founded in Berlin by a group of private bankers who want to create an alternative to UK banks that dominate the financing of trade. A group of merchants and bankers in Hamburg have a similar goal when they found Commerz- und Disconto-Bank. Over the coming decades, the banks play a crucial role in funding the development of German industries from electrical engineering to chemicals.
1914: German newspaper "Frankfurter Zeitung" reports that Deutsche Bank is the world's biggest bank. World War I temporarily cuts off economic globalisation and forces both banks to focus on their home country.
Great depression
1929: Commerz- und Privat-Bank combines with Mitteldeutsche Creditbank while Deutsche Bank ties up with its biggest competitor, Disconto- Gesellschaft in October of that year. The same month, the New York stock market crashes, presaging the Great Depression.
1933: Adolf Hitler's National Socialists seize power in Germany and the banks soon push out Jewish executives and staff. Five years later German banks assist the state in systematically freezing and expropriating the assets of their Jewish clients.
War years
1939-46: The banks take advantage of Germany's aggression against other countries to open branches and acquire competitors in occupied territory during World War II. Deutsche Bank also plays a key role in helping the German Reich fund itself by purchasing gold from the Reichsbank and selling it in Istanbul. Some of that gold came from Holocaust victims, although studies have shown there isn't a definitive answer to whether the bank knew that.
1957-1980s: Deutsche Bank is reformed and prepares to return to its role of international financier. The following year, Anglo American Corporation of South Africa taps the bank to float the first foreign bond on the German capital market since 1914. The banks support Germany's economic boom with everything from helping Volkswagen AG and other companies sell shares to financing the country's energy imports. In the 1970s and 1980s the banks expand internationally, open branches across the globe and buy competitors.
1989-1999: Deutsche Bank moves in 1989 to acquire London-based Morgan Grenfell to become a global investment bank. It's the first big step in an aggressive two-decade expansion that will come back to haunt the firm after the financial crisis. In 1999 it acquires Bankers Trust Corp. to expand in the US and briefly becomes the world's biggest financial services company. Goldman Sachs banker Paul Achleitner advises.
2005: Commerzbank leapfrogs Dresdner Bank to become Germany's second-biggest publicly traded lender with a deal to take control of mortgage lender Eurohypo for more than €4.5bn. Eurohypo's portfolio of US subprime mortgages, commercial real estate and Greek government bonds will later saddle Commerzbank with billions of euros of losses.
Financial crisis
September 2008: Commerzbank agrees to buy Dresdner from Allianz SE for €9.8bn to leapfrog Deutsche Bank in terms of German customers and branches. Lehman Brothers Holdings Inc. collapses two weeks later, sparking a global credit crunch. Later in 2008 Commerzbank plans to tap Germany's 500-billion rescue package for €8.2bn. Deutsche Bank forgoes direct state aid. Still, the bank uses the US Federal Reserve's emergency lending program.
2009: Commerzbank taps Germany for a second bailout. The bank receives a €10bn capital injection and hands the state a 25% stake in the company.
Capital raising
2010: Deutsche Bank buys German consumer lender Deutsche Postbank AG to counterbalance its reliance on investment banking. The bank raises €10.2bn of fresh capital from investors to fund that transaction and bolster its financial strength.
2011: Europe's sovereign debt crisis is raging and Commerzbank takes about €2.2bn of writedowns on its holdings of Greek bonds.