Attrition
From EU3 Wiki
[edit] Attrition
Attrition occurs when either the number of troops in a province is too large for the Supply limit or you are in an enemy province during the winter months.
If attrition is applied is calculated on the basis of the number of troops in a province, not the number of regiments. Every 1000 troops require 1 supply limit. So 1 regiment of 250 and 1 regiment of 750 require only 1 supply limit. In tropical provinces 5 is added to the required supply. For example: 1000 troops in a tropical province would require a supply limit of 6 to avoid attrition. The tropical penalty is not applied if your country's capital is in a tropical province.
If the number of troops is greater than the supply limit than attrition is applied to the troops on the 1st day of the month.

