Thu Feb 20, 2014 8:47 am
Durk has it in a nutshell, but if you want more detail:
If forces in a region do not have enough general supply they will attempt to live off the land. Each element in the region makes a check to see if they can gather enough food to survive. In game this is referred to as Foraging. If they succeed, they get enough food to eat for one turn. As soon as an element fails, no further elements can live off the land and the region becomes pillaged. (Elements are the NATO icons on the right hand side of the screen when you click on a unit in a unit panel. Many units have more than one element.) Pillaged regions do not produce supply from the land, though structures continue to produce supply normally (in most AGEOD games; have to double check for WIA vis a vis pillaging and structures). Regions usually provide 0, 1, or 2 supply chits (without ammo) depending on terrain and weather, but will not produce any if pillaged. (A supply chit is worth 5 supply points, which is what a unit's consumption is measured in. As an aside, a supply crate on the map holds three supply chips, so represents 15 supply points each.) Pillaged regions eventually recover.
Some units have the Pillager special ability. They automatically pillage enemy regions, but you almost can't help pillaging anyway, so not a big deal, and it only denies the enemy 10 supply points at most (non-negligible, but still small). They will destroy undefended depots and stockpiles of supply that they come across (in regions they pass through, but not their final destination). Unfortunately this is not that useful either, you won't see many ungaurded depots and supply does not carry over between turns. Destroy some supplies and the next turn the structures will refill them back to their max amount anyway. Also, be careful not to send them into regions with no MC that you are planning on marching through with other troops soon, you might not want that region pillaged.
In later games all units pillage enemy regions they transit when set to Evade Combat. Not sure if this is the rule in WIA, or if only Pillager units do it. Try it out in one of your games and let us know how it goes.