Cracked has a fun list of seven crazy but real conspiracies.
#7. The Business Plot
The Plan: In 1933, group of wealthy businessmen that allegedly included the heads of Chase Bank, GM, Goodyear, Standard Oil, the DuPont family and Senator Prescott Bush tried to recruit Marine Corps Major General Smedley Butler to lead a military coup against President FDR and install a fascist dictatorship in the United States. And yes, we're talking about the same Prescott Bush who fathered one US President and grandfathered another one.
How did that work out?: A good rule of thumb: never trust a man named Smedley to run your hostile military coup for you. Besides being no fan of fascism, Smedley Butler was both a patriot and a vocal FDR supporter. Apparently none of these criminal masterminds noticed that their prospective point man had actively stumped for FDR in 1932.
Smedley spilled the beans to a congressional committee in 1934. Everyone he accused of being a conspirator vehemently denied it, and none of them were brought up on criminal charges. Still, the House McCormack-Dickstein Committee did at least acknowledge the existence of the conspiracy, which ended up never getting past the initial planning stages.
Though many of the people who had allegedly backed the Business Plot also maintained financial ties with Nazi Germany up through America's entry into World War II.
