And, yes Sterling, and subsequently, the Dollar have been global reserve currencies ultimately backed by the ability to project power globally.
But money/currency may always involve - irrespective of any "official" currency - a consensual two way process.
If both parties agree - then it's money - whether "IT" is cowrie shells, salt, gold, whatever.
In my opinion energy units are the most obviously fungible value units across borders, and land/property rental units the most obvious value units domestically.
For a monetary system all that is then required is a generic accounting system/transaction engine and a suitable guarantee mechanism for bilateral credit.
In essence, the result is the International Clearing Union proposed by Keynes at Bretton Woods.
If people are prepared to accept energy units as currency, then what the government does is a matter of indifference.
Except that you will still have to pay your taxes in the governments currency. And that is backed by violence and jails. And money from the government will be in the governments currency. That gives the governments currency an advantage over any alternative currency. And then there is of course inertia.
I just realised that I dreamt last night that Sweden had changed to euros and old swedish bills were useless. Pretty, but useless. Dreams are odd things.
