The biggest false flag operation in human history will be when the aliens officially make contact, or rather when we use high tech subterfuge to pretend aliens have made contact.
“Perhaps we need some outside, universal threat to make us recognize this common bond. I occasionally think how quickly our differences worldwide would vanish if we were facing an alien threat from outside this world." President Ronald Reagan, 1987.
I'm not saying extra dimensional beings do not exist, I believe they do, but I don't think they will make themselves known in the material plane on a global scale. I doubt we could shoot down alien tech, but we could definitely down some pretend alien tech.
They have been, and always will be. 10,000 years ago we called them gods, 2,000 years ago we called them angels and demons, and today we call them aliens. Same entities, different cultural perspectives depending on the time they were witnessed.