Regiment

From EU3 Wiki

The regiment within EU3 is the basic land military unit that consists of 1,000 soldiers. Each regiment will be classified as either infantry, cavalry or artillery.

There are two classifications of regiments which can be recruited through the course of the game, mercenaries and levies. Levies are troops which are trained over a period of time and then join your armed forces. They deplete your manpower pool by 1000 and always cost the same adjusted price. Mercenaries are the other type of troop that you can hire. They do not deplete you manpower pool, but are more expensive to recruit/maintain in large numbers and give less land tradition in battle.