To me it was WW2 and the lingering "intelligence apparatus" it spawned. We went from using our resources for national security and started using it to steal banana plantation land and contracts in South America. It went from a necessary evil to a clandestine service available to the highest bidder.
Yes, it was Franklin Roosevelts declining health and death the fascists took over. They already brought Hitler to power, and from then on took over their state. They planned, but didn't need their Business Plot.