The concepts of underpopulate and overpopulate

To start talk about war strategy everyone need to understand the concepts of underpopulate and overpopulate.

There are basically two ways to cause real damage on a province:

UNDERPOPULATE

The idea of underpopulate a province is damage his economy. The focus is on killing his peasants to the point that he can no longer afford his military. It can be specially good against provinces with strong economy and possible with attackers that grew a lot during the war. What to do to underpopulate a province:
1 - Fireball: It is the main artifice to underpopulate a province, killing direct the peasants.
2 - Kidnap: Work similar to fireball, but it is not so efficient, should be used mainly if you in need of peasants.
3 - Storms: to constantly kill the peasants.
4 - Chastity: Very important keep chastity active 24 hours a day on provinces underpopulated, it will prevent the growth of peasants.
5 - Greed: Since we are damage the economy of the province underpopulated, greed will help to the province don´t have the possibility in pay the wages of his army
6 - Explosions: In lack of gold to pay his army, an underpopulated province will ask for resources. Explosions will eat some portion of this resources sent to him.
7 - Meteor Shower: Helps to kill the peasants.
8 - Free Prisoners: If the province has a good amount of prisoners, free them helps to decrease his economy, remembering that each 2 prisoner fills 1 job, and every prisoner produces 0.5gc per hour.
9 - Expose Thieves: One way to a player repopulate his lands is kidnapping, so if the province has a strong thievery power, is a good idea expose his thieves to preclude this action.

Attacking an underpopulated province:
We should never take land from an underpopulate province, the idea is exactly the contrary: the province should become fat, with lots of land but no peasants. The more land an underpopulate province has, worse his situation. Acres without peasants only means less building efficiency, less bonus from science and the need for gold to build the undeveloped land. So we NEVER should take land from an underpopulate target. The best option to attack these targets is massacre, killing their peasants, and also killing wizards and thieves, helping our mages and thieves to cause damage on their lands.

A province need to reach less than 1 peasant per acre to be really underpopulated and don´t generate gold.


OVERPOPULATE

Overpopulate is probably the best option to destroy a province. Overpopulate means take the land from a province to the situation in which it does not have room to support his troops, causing desertation. We overpopulate a province chaining it with attacks that take its land.
Link here 
While underpopulate strategy is mainly done by T/Ms, chain a province to overpopulate it is the function of mainly our attackers.
Essentially it is done with a wave of Traditional March focused on a single province, till it reach at least half (semi-chain) or one third of its original size. 
The T/Ms will help the chain with some spells and opperation, it will be better treated in a specific topic for "chainining".

Unlike the underpopulate strategy, when we are overpopulating a province we should NEVER kill its peasants! The more peasants the province has, more troops will desert! So, is a huge mystake do massacres or fireballs/storm on an overpopulation target.