the preservation of human culture is one of our primary directives

Isn't it all but confirmed that the Sirens aren't really the "bad" guys? Quotes like this flat-out confirm they're not enemies of humanity. We've seen (one of) the mysterious, actual enemies in Shinanos event. As far as I understand everything plot so far: The Sirens try to cultivate us, make us stronger, simply train us for the things yet to come. I don't know in which timeline (the Ashens one?) the Sirens and us were wiped out. And their goal is to prevent that.

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And Operation Siren fits nicely in this as you said. It's also a nice way to have Sirens in our port in the future. I feel a big shift is coming soon. With the previous russian event and all.