Armies in Warhammer 40,000 aren't meant to be static. Like the game world itself, your army is meant to be a developing force, with new recruits and veteran soldiers sharing the battlefield, learning and adapting and improving as they experience new horrors of war. To simulate that, Warhammer 40,000 features the Crusade system, a campaign mode that lets you track and level-up your army across several narratively connected games.
When you choose to play in Crusade mode, your miniature collection becomes your Order of Battle. Well, technically it's your Crusade Force, but for simplicity I'll call it an Order of Battle for now.
Your Order of Battle is the resource pool that your in-game superiours (the Imperium, or the Ruinous Powers, or Tau Empire, or whatever's appropriate to your army) make available to you. So if you want to send a handful of Skitarii and Vanguards and Kataphron Destroyers into a space hulk for a game, then you take those units from your Order of Battle and field them on the table. After the game, the units go back into your Ordero of Battle so they can be called into action again later. In the next game, maybe you decide to send some Vanguards, a few Ironstrider Ballistarii, and some Kastelan Robots to secure a city on a nearby planet, so you take those units from your Order of Battle. And so on.
In real life, this is no different from taking toys out of the various drawers or boxes where you store them, but in Crusade mode these units get recorded on a Crusade card. For every battle, you earn Requisition Points and Experience Points.
Experience and battle honours
Experience Points earn a unit a Rank, from the lowly Battle Ready to Blooded, Battle-hardened, Heroic, and Legendary. With each rank, you can choose a Battle Honour (special bonuses in the categories of Battle Traits, Weapon Modifications, or Crusade Relics) for the unit.
Battle scars
Of course, battles are also dangerous. At the end of each game, you must take an Out of Action test for each unit that got destroyed. If you roll a 1, the unit either loses a Battle Honour (assuming it has one) or takes a Battle Scar. A Battle Scar, as you might imagine, is as bad as a Battle Honour is good. The unit might lose a point from its Objective Control (OC) stat, or it may become immune to Aura abilities of an ally, and so on.
Requisitions
When you play a Crusade game, you earn a Requisition Point (RP), regardless of whether you win, lose, or tie. Requisition Points are a little like Command Points that persist beyond a single game. You can spend RP on benefits for, and maintenance of, your Order of Battle. Most notably, you can heal those nasty Battle Scars with RP. You can also increase the limits on the size of your Order of Battle, or the rank a unit is able to acquire, and so on.
Terminology overload
When you choose to play in Crusade mode, your miniature collection becomes your Crusade Force. Your collection of a specific faction becomes an Order of Battle. In other words, your Crusade Force is all available miniatures, while each Order of Battle is a grouping of a faction. This hierarchy isn't very meaningful, and I admit I'm confused why both a Crusade Force and an Order of Battle exists when the only meaningful mechanic is that your units must be recorded on a Crusade Card.
Play as the Departmento Munitorum
You may never have realised that you wanted to play Warhammer 40,000 as a Departmento Munitorum accountant, but that's what you get to do in Crusade mode, and it's a lot of fun. Tracking the progress of your units feels like watching your RPG character develop. Admittedly, there are a lot of special rules that apply to units in Warhammer 40,000 already, and adding yet more things to remember can be a burden if you're still learning the game or how your army plays. But once you've got the rules internalised, adding in the Crusade benefits and penalties is relatively easy to remember.
The really fun part about a Crusade is that choosing which unit to send into a battle becomes a question of immediate need and long-term strategy. Do you send in the veteran unit in hopes of a swift and sure victory? Or do you risk sending in a unit of fresh recruits in hopes of developing their skills and experience? In Crusade mode, your units aren't just points to be used up in your army roster (although they are still that, because Crusade uses points just like a regular game). Every unit is a sort of character (not literally, there are still actual Character models with special benefits) that you look after and guide and grow and, hopefully, empower.
Narrative play
This is reflective of what Crusade's striving to do. Crusade is meant to be the "narrative" mode of play for Warhammer. It brings elements of roleplay to Warhammer 40,000, encouraging players to come up with a story reason for battles, and to let that story effect their army in a meaningful way.
It's a little lofty to call this roleplaying. After all, you're fielding 30 or 50 or 60 miniatures on the battlefield and you maybe know the name of one of them. But then again, you are making choices about the forces you deploy, as if you were a general or a warlord. And the story behind your games is influenced by whether your army wins or loses. It might not be the same exact kind of roleplay you do in a pen-and-paper RPG, but it's not not the same kind of roleplay either. You're tracking progress, you're leveling-up, you're taking bonuses and penalties, you're participating in a shared story.
Crusade is a fun way to play Warhammer 40,000, and a great way to make the game feel a little more personal.
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