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I also took issue with the implicit assumption that two civilizations cooperating would develop no faster than a single civilization.I don't remember that assumption; I felt the implicit assumption was that development of any civilization can experience unpredictable rapid acceleration, so even if two civilizations cooperated, it doesn't help them much against others out there.
> Or, another way, that all civilizations would climb the same technology ladder the same way.
That I think was explicitly denied in the basic axioms of Dark Forest theory - again, civilization's technological level can jump unpredictably.
> historical competition between nation-states has suggested a very different reality
The core axiom of Dark Forest theory is that lack of FTL makes communication delays be of the same length as periods of possible technological leap-aheads, which makes cooperation unlikely and contact too dangerous. On the scale of our planet, that never applied. Communications between civilizations that knew about each other were on the order of months, and technological developments tended to make that delay shorter.
Dark Forest translated to Earth would be as if I noticed your sail ships, sent you a hello from my steam civilization, only to get preemptively hit by an ICBM you've managed to build in 200 years it took for my message to reach you.
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Maybe there was more historical politics in this book than I noticed; I'm not well versed on history of China, and thus lack necessary context. I'd love to learn more, if that is so.