At the beginning of the Iraq War, and for sometime thereafter, we were led to believe that Iraq was actively developing WMD. We weren't invading a country, but saving the world from imminent catastrophe at the hands of an unhinged and crazy dictator.
The proof included claims by dozens of different intelligence agencies, testimony from high level insiders in the Iraq government, surveillance photos of mobile biological weapons laboratories, definitive proof of purchase of Uranium for nuclear weapons purposes, under oath testimony, and of course endless propaganda:
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Washington Post - "Irrefutable" : https://www.washingtonpost.com/archive/opinions/2003/02/06/i...
NYTimes - "Irrefutable and Undeniable" : https://www.nytimes.com/2003/02/06/opinion/irrefutable-and-u...
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To imagine all of this was fabricated or based on lies would make even the most enthusiastic conspiracy theorist look at you a bit funny. These events played a major role in shaping my worldview. And what we're going through today is even more extreme.
The one bright side of this is that if we can manage to avoid nuclear annihilation, I expect an even larger chunk of the population will join the jaded and cynical asshole family. It's not a pleasant worldview, but probably necessary for the survival of humanity in the age of mass media and mass weapons.