Attrition
In general, armies are not subject to formalised attrition. However, in special cases there may be an attrition event run after the battles, with a chance of losing land military points or other penalty. These circumstances are:
Winter - Fighting battles during wintertime (the December-February turn) is not advised! Your troops will be extra vulnerable to attrition in the harsh conditions, if you force them to fight.
Distance - Fighting to far from your home is likewise not encouraged. Any army a large number of provinces away from its own nation will suffer attrition as supplies from home gets harassed, and the local population cannot be expected to simply allow the army to live of the land.
Massed Forces - Soldiers must forage and pillage the lands through which they pass in order to survive. Extremely large armies will suffer starvation, disease and desertion, depending on the size of the force and the value of the province. Note especially allied forces.