The government's monopoly of force and the stability of its currency are not strongly correlated. A country like Somalia had a stable currency despite the fact that its notional government barely exerted any power beyond the walls of its offices, and a country like Zimbabwe has seen its currency lose any value despite the government lacking any reasonable prospect of invasion and the military/security forces preventing any reasonable prospect of internal rebellion.

If anything, a powerful standing army is more likely to result in failure of fiat currency (hyperinflation)--hyperinflation is fundamentally caused by a government pursuing really bad economic policies to the detriment of the economy, and the failing economy will tend to precipitate an internal (potentially violent) rebellion if there is space to do so.