Themed Armies
A themed army in 40k is one that intentionally limits itself in some ways to bring out a particular idea (a Theme, if you will :) ) This may be by only using certain models or certain parts of the Force Organization Chart. (for example, a recon army focusing on Fast Attack and highly mobile Troops, ignoring Heavy Support choices, or an All Infantry list that uses no vehicles.) Alternately, it may be by converting some or all models in the army. (For example, an Imperial Guard army that is converted and painted to look like a football team.) For some interesting Themed Army ideas, check out The Jungle.
Because they are intentionally limited, Themed Armies tend to be less flexible and can be harder to win with than standard armies. Even a Themed army that uses the standard choices and has all it's options available, but is heavily converted is still limited because each individual model will require more work than a standard army, thus requiring more time and possibly money to complete a full army with all the options.
So, given that they tend to take more work and are harder to win with, why would anyone play a Themed Army? Two possibilities, Challenge and Style. The challenge presented by playing a limited army appeals to some people. You are breaking new ground and trying something new, or at least doing something that few other people do, which means you get to explore new possibilities and "go where no one has gone before." Style is another good reason to choose a Themed Army. There is a certain appeal to laying out your models and having people comment on them, even if you aren't a Golden Daemon Painter. Simply having an army that is unique and all your own is an end to itself, and a Themed Army is a great way to do that.
I want to make a note, however, about the difference between a concept army and a themed army. Every year the big automakers come out with various vehicles. Some of them are simply the new year's models, but others are Concept Cars. Concept Cars usually have a number of interesting or high-tech features and are really exciting and neat to look at. They are also usually not ever produced, except in very watered-down forms. Whether due to their high tech construction or the difficulties of driving something that looks like abstract art, or simple aesthetics, Concept Cars generally draw lots of looks from spectators, but aren't made for the street. Many army concepts are like that. There is an initial rush of creativity as the creator says, "Wouldn't it be cool if..." but then the idea fades, or the real world steps in and the army is never started, or perhaps never even fully conceived. My idea for the Sergeant Pepper's army would be a perfect example of this. (I might still resurrect it if Harlequins do become an option in 4th edition, but we'll see.) Most "Themed Armies" don't actually get past the concept stage. To pass through the concept stage, you have to fully think through the army and plan it out, then put glue to plastic and metal and get it started.
The first big hurdle is defining the Theme. What is the theme of your army, and why? Why would this army actually exist, or are you creating it just to be more effective at something or just to be eccentric? (The first can be perfectly fine, but if you are doing it just to be different and you can't come up with a "real-world" reason for someone to assemble the army, it won't be much of an army, I think.
As an example of how to describe your theme or not, a friend of mine was coming up with a concept army and when someone asked him about it, he described it a little bit and said that his theme was everything being S5 or less. Now, S5 or less is not a theme. No Space Marine Commander is going to say, "I want a task force that consists of vehicles and troops with S5 or less." On the other hand, a Commander might say, "I want to assemble a Mechanized Light infantry group or a Light Urban Task Force," for example. When questioned further, it came out that he was modeling it along the lines of the US Stryker Brigades which are mechanized groups based around light armored vehicles instead of tanks. Now it made more sense. With Rhinos and Razorbacks to move and support them, but without heavier vehicles such as Predators or LandRaiders, the army would have mobility and numbers that a force with heavier vehicles might not. His choice of Twin-linked Heavy Bolters for Razorback armament also makes sense. While one can conceive of light vehicles carrying antitank weapons, it fits better with the theme to leave them as light infantry support weapons. S5 or less is not a theme. Mechanized Light infantry IS a theme, Light Urban Task force is a theme. Both suggest something more than just S5 and have reasons to exist.
Once you have a clear concept of the Theme, think it all the way through. Start with the Force Organization Chart and your Codex. What does your Theme mean to each of the characters and units and vehicles within your Codex. You may choose to limit the number of Heavy Support Choices you can take, or you may restrict WHICH Heavy Support Choices to take. (For example, a Mechanized Light Infantry group might take Devastator squads in Rhinos/Razorbacks or perhaps even a Whirlwind, but they probably wouldn't take a Predator (much heavier tank) or Vindicator. Alternately, you might decide that they DO have access to all of the options, but they can't take more than 1 Heavy Support choice at any given time, due to limited resources.) Certain Themes may also restrict wargear or squad/vehicle options, so look through the whole codex to see what has to change. While I'm on the idea of looking through the codex and changing things, I want to point out that it's my preference to limit the choices available in the codex, rather than adding things that aren't normally available. While it might be cool to take Fast Attack instead of Troops, that significantly changes the codex and isn't legal according to the standard rules GW makes. You could actually play the army with your opponent's permission, but that can be hard to get in pick-up games and downright impossible in tournaments. Except among friends, don't make up your own rules too much, it just complicates the process of getting in a game.
Once you've thought through your Codex and Force Organization Chart options, then you'll need to start thinking through how your army will perform on the table. Take a look at various threats you might face, and see if any of them pose a serious problem for your army. Maybe you can take down vehicles easily, but if you are easily overwhelmed by hordes of basic troops, you may have a problem. Start first by identifying the major elements you might have to come to grips with. Heavy Infantry. Fast Vehicles. Heavy Vehicles. Lots of Light Infantry. Monstrous Creatures. Ordnance weapons. You may decide that it's not much of a threat, or you may have to come up with a way to deal with it, but give it some thought. Along the same lines, think through other threats as well. How will you deal with close combat armies? What about shooty armies? Mobility? Finally, take a look at the armies you are likely to come up against and decide which ones are threats that you'll need to counter. During this process, you may come across something that you couldn't reasonably counter without changing your Theme. You may decide that a small adjustment could be made and still keep mostly within the Theme. You might also decide that it's not worth adjusting the Theme over it and just deal with it as best you can. Either way, think it through.
Then you decide how you will show the Theme through your army. If your Theme involved primarily limiting your options, you may not need to do much to show it, because your choices will speak for you. You can still, however, add some things here and there that will enhance the theme. If your army is based around the idea of a mobile recon force, perhaps you should use cavalry markings on your vehicles and give your characters some symbolic cavalry equipment, like spurs, Jodphurs or riding crops. You may also want to give all your vehicles Whip Antennas to symbolize better communications gear. Note that, generally, small details are better than major changes. The more changes you have to make, the longer it will take you to build the army, which means you won't get to play any time soon. Now, get cracking on those ideas!
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