In Victoria 3, organizing an Army for war can be done in different ways. As the way to wage war is a hands-off experience, the armies as well are something to prepare, and not lead.
Your country’s Army Model is a type of Law that governs who makes up your military and how they are organized. There are currently four Army Models planned:
Under a Peasant Levy model, a permanent military force supported by Barracks is non-existent or minimal in size. Instead, the country may raise large numbers of conscripted levies of irregulars during wartime, led by Officers originating from the aristocracy. This model is cheap to maintain during peacetime but can get very expensive, particularly in lives, during war. Conscripted Battalions can also take a long time to organize, and provide no Power Projection that affects the country’s Prestige.
The National Militia model limits your standing army and Power Projection in much the same way as Peasant Levies but permits you to conscript a large part of your population into a well-rounded national defense force if needed.
Mass Conscription becomes available once you have unlocked the Society “tech” Nationalism. It lets you enlist the same large number of conscripts as National Militia but does not limit the size of your standing army, and all these conscripts are assigned under Generals as needed. In addition, conscripts are trained faster but are limited to infantry troops only.
A Regular Army model lets a country maintain an army of any size even in peacetime. While it also permits for conscription of a smaller number of civilians as needed, its standing army is the pride of the nation and confers considerable Prestige onto it. This model has the greatest flexibility in terms of military Production Methods, letting countries configure their armies without restrictions. Technologically advanced countries relying on more expensive crack troops rather than sheer numbers favor this Law.