I found this in a comment on reddit, I think it's a great answer to the question “How can something like this happen!?”
TLDR: The answer is nothing brilliant, they just pull the same old techniques from the playbooks.
Mass indoctrination
1 - Start while they’re young.
2 - Create the illusion of political freedom.
3 - Use simplistic stereotypes to sway public opinion.
4 - Mix facts with lies.
5 - A big lie is more convincing than a small lie.
6 - Give the masses “bread and circuses” to keep them well-fed and distracted.
7 - Simplify complex issues by portraying them as dichotomies. Eliminate nuance.
8 - Spread propaganda by all means possible.
9 - Ostracize dissident voices through ridicule or defamation.
10 - Faith in the correctness of a religion or ideology is more powerful than force.
11 - Manipulate history records to support your religion or ideology.
12 - Control different sides of the same debate and you control the outcome.
13 - The masses are less swayed by reason than by stirring their emotions.
14 - Drive the opposition in a corner. When they fight back, act like a victim.
15 - Label all non-conformistic behavior as pathological and promote “cures” for them.
16 - Use rituals and mass events to keep people occupied and strengthen their faith.
Propaganda Techniques
Name Calling / Stereotyping - Labeling a person or idea with an easy to remember pejorative name used to make us reject and condemn without examining what the label really is.
Virtue Words / Glittering Generality - Accepting and approving of things without examining the evidence carefully.
Deification - When ideas are made to appear sacred or above the law so that alternative ideas are given the appearance of blasphemy or treason.
Transfer /Guilt/Virtue by Association - When a symbol that carries respect is used along with an idea or argument to make it appear acceptable.
Testimonial - When a respected celebrity (or alternatively someone generally hated) claims that an idea or product is good (or bad). This technique is used to convince us without examining the facts more carefully.
Plain Folks - To convince that an idea is favorable because a vast majority of people like yourself share the same ideas and agree.
Band Wagon - When the speaker tries to convince us to accept the point of view or we will miss out on something really good.
Artificial Dichotomy - Claims that there are only two sides to an issue and that both sides must have equal presentation in order to be evaluated. This technique is used to dupe us into believing there is only one way to look at an issue, when there will always be many alternative viewpoints or sides.
Hot Potato - An inflammatory (often untrue) statement or question used to throw an opponent off guard, or to embarrass them. The fact that it may be utterly untrue is irrelevant, because it still brings controversy to the opponent.
Stalling / Ignoring the Question - Used to play for more time or to avoid answering a pointed question.
Least of Evils - Used to justify an otherwise unpleasant or unpopular point of view.
Scapegoat - Used with guilt-by-association to deflect scrutiny away from the issues. It transfers blame to one person or group of people without investigating the complexities of the issue.
Cause and Effect Mismatch - This technique confuses the audience about what is really cause and effect. In fact the causes of most phenomena are complex, and it is misleading to say just one thing caused another thing.
Distortion of Data / Out of Context / Card Stacking / Cherry Picking - Used to convince the audience by using selected information and not presenting the complete story.
Weak Inference / False Cause - When a judgment is made with insufficient evidence, or that the conclusion does not necessarily follow from the evidence given.
Faulty Analogy / Slippery Slope - When a comparison is carried to far. Slippery slope makes the argument that a shift in one direction will continue to lead to extremes, when it is not necessarily so.
Misuse of Statistics - Leaving out important parts to distort the data
Fear - Voice or no voice, the people can always be brought to the bidding of the leaders. That is easy. All you have to do is tell them they are in danger.
Ad hominem Attack / Deflection - Attacking the messenger, instead of the argument or evidence that is presented.
Tu quoque Attack (too-kwo-kwee) - responding to an opponent by accusing them of committing a logical fallacy or propaganda technique instead of addressing the claim of your opponent's argument and evidence.
Preemptive Framing - Framing an issue before other people get a chance to.
Diversion - When a major issue comes up that is embarrassing or threatening, a diversion is created so attention is directed away from the issue
Source: https://www.reddit.com/r/politics/comments/9xvzt8/why_the_right_is_so_obsessed_with_alexandria/e9vmtyr/?context=1