After World War I, Germany was forced to pay all of the war reparations after the Treaty of Versailles. These reparations along with the economic climate within Germany during this time period led to the fall of the Weimar Republic and the rise of Adolf Hitler and his Nazi Party.
The reconstruction period was based on private investment and demand. When the stock market crashed in 1929, the investors who had been financing Germany pulled out, crippling its economy.