Things we believed during the Cold War that ended up being wrong

Communism was a single, monolithic entity
At the height of the Cold War, it was generally agreed that having any form of Communist government in your neighborhood was like inviting Moscow to set up a missile launch pad on your front lawn. This attitude only made sense through a prism of paranoia. Rather than being a single entity dedicated to the destruction of democracy, the Socialist nations were extremely diverse and far more likely to spend time undermining one another than undermining Washington.