Heres a few conspiracy theories that turned out to be true:
7 Insane Conspiracies That Actually Happened - Democracy Forums
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.
For years, Britain had a spiffy trade deal with Iran regarding their prodigious oil fields. The Anglo-Iranian Oil Company was basically a giant money machine for the Anglo half, while the Iranian half got shafted. That all changed in 1951 when Iran nationalized the AIOC and the Iranian parliament elected Mohammed Mossadegh as Prime Minister. Mossadegh was relatively secular, something that pissed of Iranian clerics, but he was also very nationalistic. When Britain tried to regain control of the AIOC, he gave them the finger. Tea was spilled, crumpets were dropped and monocles everywhere popped out in shock.
You can guess what happened next. Jolly old England went to its ally, the United States, and convinced President Dwight D. Eisenhower to help overthrow the democratically elected leader of Iran and install a pro-West monarchy. Together the CIA and British intelligence services funneled guerrilla troops, anti-Mossadegh propaganda and tons of bribes into Iran.