And armies. Why doesn't some gang just march into a few of the big hydro-powered ASIC mining centres and take over control, thus enabling them to steal all the bitcoins. What prevents that? The armed power of nation-states to enforce a generally-agreed order on societies.

> thus enabling them to steal all the bitcoins

Just FYI, a 51% attack doesn't allow you to "steal all the bitcoins"

All that would accomplish is taking control of the revenue generated while they are in control. If the big mining centre is working with private keys that are unknown to the operators, then they would not be able to spend the coins previously mined. If they gain control of more than 51% of the hash rate in this way, they would be able to censor transactions or reorg the chain. That would still leave most of the blockchain in tact. A deep reorg might even get forked away from by the rest of the network, leaving them alone in a situation analogous to Ethereum Classic.

That's a pretty roundabout way of thinking. Plenty of countries have troops but an unstable currency (e.g. Venezuala). Plenty have a stable currency but no significant troops (e.g. Iceland).

Militarily robust sovereignty is necessary but not sufficient for the integrity of a fiat currency.

Governments such as Iceland borrow military power through international agreements.

Robust against what?

Threats to national sovereignty such as invasion.

I don't really see how protection against invasion helps you maintain a fiat currency. I don't think an army is either necessary or sufficient. These just seem like two different issues.

Do you have an example of a nation failing to repel an invasion yet maintaining the integrity of its currency?

You're shifting the goalposts. Lack of protection against invasion doesn't mean you're going to get invaded. Maintaining a standing army isn't protection against invasion either.

Do you agree that if a nation is successfully invaded its currency will lose its integrity?

Do you agree that defense against invasion requires an adequate military or agreements with nations with adequate militaries?

If we agree on those two claims, it seems we must agree that militarily robust sovereignty is necessary but not sufficient for the integrity of a fiat currency.

No thats actually the right way to think about this. Externalities are often misinterpreted to only include a few layers of abstraction when in reality its a much more complex and multilayered discussion and include both positive and negatives.