system
Given the following post, respond as if you're commenting on Reddit:
human
[Marvel Cinematic Universe] How did HYDRA maintain its infiltration of SHIELD for nearly 70 years?

I'm not asking how they avoided being exposed in that time (Zola answered that), but rather where all these younger people, like Sitwell, who would have been too young to even remember WWII. was it a chain reaction of "I can trust this guy; I'll recruit him as another HYDRA plant" starting with Zola and continuing to the present? Or was it done some other way? It's not easy to sell a conspiracy that robs the world of its freedom.
gpt
You start by choosing people carefully. Based on their political and moral views. You gain their trust and get to know them and make you and other hydra members most of their social circle, re-enforcing their views by sharing them, pushing them further slowly. 

Then get them to agree to the fact that the current system is flawed and that they need to make a change if they are to actually achieve what SHIELD is meant to do. They agree, suggest that there needs to be some sort of SHIELD within SHIELD that is dedicated to protecting mankind even above their own survival, that they don't care about how the world sees them or if they ever get recognition. They are the true heroes, willing to be seen as criminals for the greater good, not like the Avengers who took the easy route of adoration and celebrity status.

Once you have them on board with the idea you tell them whatever they need to know. You don't even have to tell them about HYDRA at this point. They are just a secret group within SHIELD that are willing to break the law and even the rules of SHIELD to see the goals and aims of SHIELD met. Eventually you can reveal that HYDRA weren't as bad as they were made out to be. That they wanted the same thing, but, went about it all the wrong way. They tried to enslave mankind and joined with the Nazis which was a wrong decision. It's not like HYDRA supported the death camps or the extermination of Jews. They just happened to form in the territories of an empire that was led by a man who was elected into power that did have those views.

Just like how we work within SHIELD, not following their beliefs or rules entirely, so did HYDRA work within the borders of the Nazis in the same way. HYDRA do what needs to be done, regardless of our individual desires or concerns. We sacrifice our own ideals, or own souls, our own morality to see that the world survives. We are the heroes the world deserves, just not the ones it needs right now...