It is not possible to "enforce" a "value system". True.
But we can have a value system we share that benefits us all. We can work together to improve our welfare as a whole.
> Ultimately the only defense we have is to not give other people such power.
That is untrue. We must have webs of trust, and within those webs there are power hierarchies. The trick is that those hierarchies must not be arbitrary nor permanent.
This is a problem that has been wrestled with, and solved, several times in human history. Any human structure is vulnerable to outside attack, and most (all?) vulnerable to internal decay, but they can be established and are worth establishing.
An example I can think of are the Republican Militias as described by George Orwell in Road to Catalonia, and activist groups I have been involved with here in Aotearoa.
Nothing lasts for ever - that does not mean we cannot work together for good things.
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In one group I was involved on we used to have monthly meetings, by phone, of the organising committee. We had thirty members (all activists running hot with their own opinions), made decisions by consensus. I was on that committee for five years and we went over our allotted two ours twice: Once by ninety minutes (that was a day!) and the other time by five minutes. Nobody ever felt unheard - that mattered to us.
It is possible, people have been doing it forever, the good things we make are vulnerable but we should still aspire to and achieve them