There are so many possible armies you can build for the 10th edition of Warhammer 40,000 that it can seem almost impossible to understand at first glance. In this post, I decipher the rules for army building in Warhammer 40,000.
The first thing to know, if you're completely new to wargaming, is that wargaming exists outside of Warhammer 40,000. I'm a 40k (and 30k) fan, so I'm not saying this to discourage you from buying into Warhammer, and in fact I think that Warhammer is one of the easiest ways to get into wargaming (it's what got me into it). However, you also ought to know that you can alternately get into wargaming through other systems, like the books published by Osprey, anything by Mantic Games, and models from Wargames Atlantic. Warhammer offers a cohesive and comprensive inroad to the hobby, though, and so it is a valid destination.
Army building starts on paper. Do not buy miniatures for your army until you know which miniatures your army needs. Conveniently, building an army is essentially the process of figuring out what to go out and buy.
The general rules about what makes an army start on pages 55 and 56 of the core rulebook for 10th edition of Warhammer 40,000. You can't implement any of the rules listed on pages 55 and 56 with just the core rulebook, but it's still information you need to know.
It starts with one of the most important concept you need to understand about a wargame army: The size of a legal army is entirely arbitrary, and it's defined an imaginary Build Points. For each unit of soldiers you add to your army, you "pay " some number of Build Points. If you and your opponent agree to bring 500 point armies to the gaming table, then a 500 point army is legal. If you and your opponent agree to bring 1000 point armies, then a 1000 point army is legal.
Before explaining the army building process, it's important to know that the 10th edition of Warhammer 40k has a game mode called Combat Patrol that makes building your army really simple.
That's it. In 3 easy steps, you're assembling, painting, and playing.
If you want more flexibility, or bigger armies than what fits into a Combat Patrol box, then read on.
The next step is to choose a Faction. The Faction you choose governs what miniatures and supplemental book you'll buy.
Next, at least according to the book, you choose a Detachment. A Detachment is often tailored to specific units, so it may influence which units you include in your [as yet non-existent] army. For instance, the Brood Brothers Auxilia Detachment for Genestealer Cults allows you to add some Astra Militarum units to your Genestealer Cults army, so you would need to purchase Astra Militarum units to make that Detachment work.
A Detachment also influences how you play. For instance, the Adeptus Mechanicus Rad-zone corps Detachment gives you broad control over the battlefield by infusing it with fallout that potentially causes damage to your opponent's troops. But the Skitarii hunter cohort Adeptus Mechanicus Detachment grants Stealth to all of your models with the Skitarii keyword, so you're able run boldly into battle with troops that are suddenly harder to hit than in other Detachments.
It's basically impossible to make an intelligent choice of Detachment before you're familiar with how your Faction actually plays, but then again it's impossible to get familiar with a Faction without choosing a Detachment.
But anyway, the core rulebook doesn't list any Detachments, so for now this is just a requirement to remember later: Don't go to battle without choosing Detachment rules.
Step 5 on page 56 contains the broad core set of rules about what an army can and cannot contain. An army in Warhammer 40,000 consists of units. A unit is defined in a data card published in an army index or codex, and it might be just 1 really special soldier, or 10 infantry soldiers, or a tank, or anything else the Citadel miniature designers invent. A unit always has at least 1 Faction keyword (such as Adeptus Mechanicus or Adeptus Astartes or Heretic Astartes and so on). It also usually has several general keywords, such as Infantry, Transport, Character, Epic Hero, and so on.
For each unit you include in your army, you subtract its Points cost from your total Point pool. In other words, if you and your opponent have agreed upon a 500 Point army, and you add a Character unit costing 35 Points plus an optional 10 Point enhancement, then you have (500-45) 455 Points left to spend on more units.
Here are the universal army build rules:
That's all. Now all you have to do is go choose which units you want in your army.
Units in your Faction are listed in the corresponding army codex or index.
An index contains at least 1 Detachment rule set, and a datasheet for every box of miniatures that Citadel currently sells for your Faction. Many indexes are available for $0 from Warhammer Community, although they're removed when a codex for the same Faction is published.
A codex is a physical book about a Faction in Warhammer 40,000. It contains lore, several choices of a Detachment so you can play to a specific strategy, special Strategems, a datasheet for every box of miniatures for that Faction that Citadel currently sells, and how many build Points each unit costs.
With the general rules defined in the core rulebook, and the datasheets in an index or codex, you can build a legal army. To do it, you look through the datasheets of your Faction and choose the units you like and can afford given your Points target.
Each unit is described in its own data card, which provides its characteristics (Weapon Skill, Ballistic Skill, Toughness, Save, and so on), keywords and, most importantly for buying the right miniatures, the Unit Composition. Usually found on the "back" of the data card, Unit Composition tells you how many models you need to create that unit. Generally, Unit Composition is conveniently the same as 1 box of miniatures with the same name, but that's not always the case. A unit of Skitarii Rangers happens to contain the same number of models as there are in the Skitarii Rangers box, but a unit of Kastelan Robots (sold 2 in a box) can have anywhere from 2 to 4 robots.
Write down each unit you decide you want in your army, and keep track of the Points you're spending according to the chart either in the back of the book or in the downloadable Munitorum Field Manual. Stop adding units once you're out of points, or mostly out of points. There are a few additional options, called Enhancements, that you might be able to spend some points on.
The Detachment you choose for your army features Enhancements that, according to the core rulebook, any unit with the Character keyword may take at an additional cost. There are some restrictions:
Enhancements range in cost from 10 to 50 Points, on average, so keeping your army just shy of your Points limit gives you a little flexibility here.
The final step is to choose 1 model with the Character keyword to gain the Warlord keyword. Your Warlord doesn't get anything special other than the keyword, but your Detachment and its Strategems, or the Mission you're playing, may have special rules that mention the Warlord specifically.
Once you have a paper list of all the units you want in your army, it's safe to go make your purchases. These models are the ones you're going to assemble and paint and play with. This is your Warhammer 40,000 army.
You might notice that building an army isn't entirely linear. There's a lot of uninformed decisions you have to make, like which Faction to play and which Detachment to use. But how do you make those choices intelligently before you actually know how a Faction plays and how a Detachment actually performs on the battlefield?
Many players own more miniatures than they deploy all at once. You might collect a bunch of Kastelan Robots, but only regularly deploy 2 to 4 of them, except for that rare game when you decide to play nothing but Kastelan Robots (which you can actually kind of do, using the Cohort cybernetica Detachment). You don't have to collect more miniatures than you deploy. You're free to buy just the miniatures you need for exactly 1 army using exactly 1 Detachment, or you can just buy a Combat Patrol box and use it. But if this process has seemed complex and maybe over-flexible, the reason is that most players extract an army from a larger collection of miniatures lying in wait. I don't think this is something you'd try to do right away, but I do think it's something that happens over time as you idly buy a model kit here and there. It just starts to add up. The good news is that you can reconfigure your army as often as you want, using miniatures that don't see as much action as others.