A necromancer decided to raise an army of zombies to attack the neighbouring kingdom. It's a relatively small and decentralized kingdom, about the size of Denmark. In this world, zombies cannot infect zombism to living beings. But getting bitten can still inflict other diseases like rage. The only way to create new zombies is with necromancy, a dark form of magic known to only a few.
Assuming the necromancer has access to about 10 000 fresh corpses to start the invasion. Once the invasion is started, he will not be able to create new zombies for a while because it's exhausting and he is busy waging a war. After a while it will become possible the crate new zombies to replace the decayed ones by using the fresh corpses of the countryside.
Zombies are slow and dumb but do not fear danger. Zombies do not suffer and move even if you cut their head off. Limbs that are severed or damaged enough will stop moving. If the body parts become too decayed, they will stop responding to magic. Zombies will continue to move unless all the limbs are dead or if you hack the torso in pieces. I believe the decomposition rate might be faster than on a corpse lying still because the zombies are moving. The magic makes the zombies move but it does not slow the decaying rate.
Considering the decaying rate of the human body, is this kind of invasion even possible? How long will it take before the initial army falls into pieces by itself?
Are there any natural techniques or type of environments that would slow down the decaying process?