Nonexistent weapons

Mybe I am missing something here, but how is it possible that a country, from which the defensive weapons are destroyed, is still able to lose them (again), and destroy my armies in the process?

Nope. If they are destroyed, they are destroyed.

You are probably being attacked by Stealth Bombers or Air Wings.

The other possibility is the country resupplied a unit that lost all its weapons. Make sure you are destroying the defensive units if you are only attacking defenses at cities and such.

hmm, last attack:

The country lost 16 anti aircraft missile batteries, 3 missile interceptor batteries, 15 defensive missile batteries, 60 jeeps, 60 artillery and 50 tanks.

But the only defensive units they have at the moment were 100 helicopters and a radar plane...

Maybe I am not understanding something. Are you saying that you destroyed the garrison already (the weapons deployed at cities and such) and they came back? Or are you attacking cities and, not seeing any deployed units with these weapons, assuming they are undefended?

I destroyed the weapons in the cities and they came back, probably because of the resupply units I assume,
but the list of their defensive forces states that, apart from the helis and the radar plane, there is nothing left to shoot back at me.

The clue may be in the fact that there are still helicopters around.