. . . is that every single thing that each side's propaganda machine told people about the opposing side was utterly, absolutely, and completely TRUE.
The people, though, knew that it was propaganda, and took it to mean that the opposite of what the propaganda was telling them was the case. And that was a huge mistake. We should have believed our respective propaganda machines, to begin with. Just imagine how many unpleasant surprises would have been avoided if we had.