Home > Uncategorized > Learning from XCOM, 1: the rules that matter

Learning from XCOM, 1: the rules that matter

February 7, 2023 Leave a comment Go to comments

I am not the first person to notice that XCOM is a very good computerization of a tabletop miniatures wargame.

Nor am I the first to notice that playing video games is actually pretty useful for understanding the workings of tabletop/analogue games. It lets you engage with the rules without the noise of table interaction* so you can see what those rules really do. You can play through a hundred encounters very quickly, so you can see what behaviors and tactics emerge once you’ve grokked the system’s affordances.

So having sunk a few hundred hours into the XCOM series, I have some blog posts’ worth of realizations from it.
This first one is about the basic combat rules of its tactical game – when your minis are running around and shooting.
Because I for one find it difficult to design fun combat, and XCOM makes some really good decisions that I can learn from.

A very brief introduction: XCOM is a long-running series of games (canceled, rebooted, remixed) that started with UFO: Enemy Unknown in 1994. They’re pretty consistent except for the most recent Chimera Squad, which I won’t be discussing because it’s a radical departure and also not very good.

The plot: aliens are invading and abducting people exactly in the style of 1950s monster movies. You play the perpetually underfunded special agency that’s supposed to stop them. So the game consists of waiting in your base for the aliens to strike, then scrambling aircraft and small ground squads of 4-6 soldiers to stop them.

The game: consists of two modes – strategic, where you upgrade your base and troops and try to find out what the aliens’ plan is, and tactical, where you skulk around gas stations and graveyards getting shot at by raygun-toting greys and knock-off Martian tripods, resolved in turn-based combat on a square grid map, just like a dungeon.

so you start strategic – learning about a mission/dungeon at your base…

…avoid the wilderness travel sequence with a dropship montage…

…and move to tactical mode when your squad hits the ground.

Every part of this 2-layer experience is designed to ratchet up the tension – if you build the wrong facilities, research the wrong technologies, or get wounded or killed in the field then the aliens get closer to winning. If you want to know what the emotional beats of this are like in play but don’t have time to play the game, this guy has done an astonishing job of documenting them.

People keep comparing XCOM to D&D and… sure, you could do that The squad size, the character classes, the leveling up (each of which will get its own blog post in time) are obvious commonalities. Those aliens could be liches instead. But as a setting it’s a lot more like Delta Green: the enemy really is unknown (at the start), they keep springing gruesome surprises, and not all your shiny soldiers will be coming home. It could also work in Classic Traveller – the characters are pretty flat and interchangeable when they join you: their stories develop through play. And the whole thing is soaked in Trav’s space Vietnam assumptions.

OK, so. I was going to write about combat.

What XCOM gets right about tactical combat and how it’s useful for RPGs.

Disclaimer: there are small variations in the rules between versions – for the purpose of this discussion I will blend them all together into a sort of ur-ruleset that suits my arguments. I may write a post later that breaks out the differences, because it’s an interesting comparative case for RPG edition wars.

It’s tempting to write computer games off for TTRPG design by saying “well if you have a computer doing the heavy lifting, of course you can afford all that fun crunch that… would be fun if it didn’t slow the action down to a crawl.” And yes, XCOM does benefit from mechanization, but don’t get distracted, it also does a bunch of smart things that aren’t about computerization.

Most of all, it’s absolutely ruthless about only including calculations that the player can make choices about. If the player can’t exploit a factor to shift the odds in their own favour then XCOM doesn’t model that factor.

And it knows that meaningful decisions require information at the right time, so it shows the player its work – it shows the calculations that go into its model and rolls its dice right out in the open and, most importantly, it tells the player exactly what they need to roll to pull off a shot before they’ve done it, so they can change their mind and try something else.

Here we’re looking over the shoulder of one of your soldiers – a Scot – and they’re targeting an alien – presumably a sassenach. You can see right there (you might have to click to embiggen): 75% chance to hit. And they will do 3-5 damage if they do hit. Since we know the enemy has 3HP (those 3 bars above their head), that’s a guaranteed kill. More on that later.

And here are the factors that go into the calculation: your soldier’s chance to hit based on class/training (68) +20 for height advantage (because you deliberately went up to high ground to get that bonus) +5 for scope (special equipment you chose) +pfeh for range (we’ll be simplifying that) -20 for cover (because the enemy was smart enough to hide behind a wall).

All the factors in the picture above reflect tactical decisions: training, equipping, choosing positions. If you’re in the game and want to boost your chance to hit further you could:
– destroy the enemy’s cover with a grenade,
– change weapons to something with a bigger bonus, or
– choose another target who isn’t in cover (looks like there are 9 enemies in view – not good! But at least one of them’s probably out in the open).

Since you have a squad, there’s a higher level of choice, too, based on the squad members supporting each other, concentrating fire against a single enemy, or guarding each other’s backs. Who among your soldiers will move first? Who takes each particular shot? Who has the greatest flexibility to shoot someone else, if the current target gets taken care of?

Those choices are the fun of the game, so the game puts them front and center. And it highlights the available choices at every step – it’s very good at letting you know what you know and what you cannot know. For instance, if enemies pass out of view, they disappear from the map. If you can’t target them, there’s no little red head icon over the Fire Weapon tab. And when enemies enter the fight – when they first notice you – they interrupt your action with a “wrestling ring entrance,” to highlight where the threats are. Those flourishes play to the affordances of the computer, sure, but something like them can be approximated with minis and chits or a whiteboard app for those playing on zoom.

The field of choices is kept manageable by rigid constraints. In one round each character can do 2 actions – move + move or move + shoot/do other action. Shooting always ends your round, even if you haven’t moved. And instead of shooting now, you could hold your fire (“overwatch”), hoping the enemy will break cover on their turn so you can hit them with a reaction shot. The rigidity provides the solid arena on which you can base your actions. You can tell at a glance whether an enemy is close enough to be stabbed – or for them to stab you on their turn. You can reckon their action economy and take calculated risks. And you can think architecturally about the combat – choose which enemies must be killed before they can retaliate and which you can afford to have survive for another round; decide where to concentrate your force; when to fall back and who will be vulnerable when you do so.

There’s one of your soldiers. The yellow line shows which squares they can move to within 1 action. The cyan half-shield means they’ll get partial cover if they go to that square. Is that good cover? Depends which direction the aliens are coming from.

This is a bad position for your guy (identified by blue HP markers – enemies are red) That musclehunk thing is close enough to charge and punch him (note how the battlegrid is subtly shown by the pavement tiles) and it has way too much HP for your guy to kill it with one shot. OTOH the musclehunk would have to go through the poison gas, so maybe that’s a lure. BTW the wee full shield next to your guy’s HP meter shows he’s in full cover – both from the signboard he’s hiding next to and from the wall behind him.

The choices can get pretty complex, but they mostly boil down to take more risk to end this now vs. take less risk and try to improve your position for next time. If you ever get in a situation where you’re just standing toe to toe with the enemy trading shots – the stereotypical bad combat experience that people complain about in D&D – it’s because you’ve either had a failure of imagination or you’ve got into a terrible position, from which you cannot reach a place of advantage. It’s a failure mode and you can see that because of XCOM’s clarity.

If the player cannot make meaningful choices about something, XCOM doesn’t care about it either. The big one here is a D&D mainstay – the swingy damage roll. XCOM doesn’t do them. The randomness is already in the to-hit roll and it’s no fun to hit, only to find that the hit doesn’t count. So damage is predictable. Sometimes there’s a little variation, because that little bit of risk – will this shot kill or not quite? – gets the heart racing. And unexpected windfalls are always welcome, so you can still get crits and unexpectedly kill a tough alien in one shot. But for the most part – especially for area-effect weapons (grenades) – it’s a set number of HP. If you’re in the blast radius you get hit, no exceptions. And it works. I don’t think “oh but partial cover!” I don’t miss the damage roll. At all.

The main consequence of damage being a constant rather than a variable is that it makes fights faster and more fluid. Think of it like this: a fighter with a 50% chance to hit only advances the combat once every 2 rounds. They’re like half a fighter, unpredictably doing nothing half the time. The same is true of a fighter who hits all the time but sometimes does so little damage that it makes no difference. Moreover, until that fighter has resolved the uncertainty of whether they will make themselves relevant this round, the threat of the enemy they may or may not kill is still there, tying up other characters. You can’t think past the current enemy, because the uncertainty is so great that it’s not worth considering what things would look like without them. You might spend 5 rounds plugging away at them rolling 1s for damage. OTOH with predictable damage, you can plan farther ahead – if you know an enemy will take 4 hits to kill, you know to concentrate lots of fire on them.
In short, more predictability = more tactical value = more mental engagement. Of course, different people have different sweet spots, between randomness and tactical certainty. Not everyone wants chess. But reading the D&D blogs I feel like not everyone wants B/X’s level of unknown, either. This is a way to shift the balance a bit.

Other simplifications:
– Armor is just more HP (because HP = luck or moxie anyway, right?). Damage takes armor off first. And armor autoheals before the next adventure, so there’s no healing downtime for those HP.
– No perception rolls – you can see everything within 12 squares unless it’s hidden behind full cover, in which case you have no line-of-sight (placing sneaking in the player’s hands, because you have to be careful with the grid).
– No rolling for initiative – aliens get their ring entrance but never get to shoot from surprise, so the player gets a turn of moving and shooting at the beginning of each encounter. That is a massive advantage – there is one game mode where the aliens always win initiative and it’s brutal – but in the interests of a fun game of PCs vs. monsters, it’s amazing how quickly you get used to that little elision, and how immediately you’re in the mode of being scared because you’re in the wrong position, rather than bored because you always go first. If you were playing 2 PC gangs against each other then it would be different, of course (like XCOM2’s multiplayer mode). It turns out that game is all about ambushes.

Complications are many, and they all consist of ways PCs can bend the basic rules. Some PCs can get a third move before or after attacking, some can fire-move or fire twice instead of moving. And there’s a ton of special moves – an extra action if you kill an enemy, automatic reactive attacks if someone passes close to you, suppression fire to disadvantage enemies etc etc etc. That will be a separate blog post, because some of them are really good and deserve special attention.

So, how can we adapt this to improve TTRPG combat?

To answer that question thoroughly, I’d have to actually get around to playing D&D4e and understanding what people don’t like about it. For now I’m just going to offer some principles, while wistfully pointing to this wiki, which – if you had a lot of time – you could scrape for all of XCOM’s rules, which you could then analyze in depth and simplify down rationally over a couple of months. Given the smart job the XCOM design team have done, I think it’s better overall to adapt their rule set and their particular numbers for HP, damage etc. rather than trying to retrofit it to B/X or something but… I know other people have a great big love for B/X, so:

  1. You gotta use a map. Sorry, so many of the tactical decisions here are spatial, and so much information is contained on the map, that it would be a very different game without it. Well, maybe there are ways – you could make “outflank” and “take cover” and “lure out of cover” into Complications, potentially. You could maybe create a theatre-of-the-mind version of this… but that’s at least another blog post.
  1. The 2-move structure allows a good level of PC agency in a round: it’s enough to get into trouble AND do something about it. With only 1 move you always end your round as a sitting duck – it’s not worth trying to get tactically clever.
    So you can move-move or move-attack, but attacking ends your turn. If you hold a reaction shot (overwatch), you just interrupt the enemy as soon as they move 1 square.

  2. Oh yeah, missile weapons. They’re better than melee, sorry. Better tactical affordances, more spatial architectural play. You can absolutely fit this to a D&D-alike, just give all your Vikings throwing axes.
    OK, I understand that’s not a popular stance, but XCOM2 has a neat solution for bringing knives to a gunfight – the swordsman class gets a sneaky 3rd action: if you charge (they say “slash”) then you can melee strike at the end of your second move. And melee attacks ignore cover, so they almost always hit (that almost is a perennial complaint of XCOM players, who get angry when they miss a 98% chance to hit and insist the RNG hates them. I’ve rolled enough dice and been saved in Cthulhu games enough by rolling a 01 to know that it does indeed happen, more often than you’d expect, evidently). It’s awesome, trust me – gets you across the map and into trouble fast. You could also invent a spear charge that increases to-hit risk both for the user and the target.

  3. Damage is a constant for each weapon (though crits are still possible). Some classes and levels could give bonuses to that constant, and those calculations can get baroque if you want, but that’s all precalculated down to a number on the charsheet.

  4. Armor is more HP. Cover is more HP. When they’re gone, they’re gone. Yup, destructible terrain, like in XCOM: you can force the enemy out by taking down the wall in front of them. If you want, some armor and cover could be Special, and give a few points of constant DR until it gets destroyed (“shredded,” in XCOM2’s terminology) by Special attacks. That’s a complication you could add for particular classes/weapons. BTW, cover only applies if you’re right next to it, not if it’s just somewhere in the line of sight.

  5. Use D10 for to-hit rolls. XCOM uses percentiles but… I would just approximate all bonuses/penalties to multiples of 10% (ie +/-1). The point here is to get player-focused simplicity and when you’re trying to decide whether to break cover for a better shot, you don’t really care about less than 10% difference. And don’t be afraid of having a 0 or 100% chance to hit – that’s how XCOM works and it’s great. You get that 100% chance because you did something smart or took a big risk (like closing to melee). As a base target to hit, I suggest you have to roll 7+, simply because there will be more ways to boost your to-hit chance than to reduce it. Oh and a natural 10 is a crit and does double damage. Unless you can’t hit even with a natural 10, in which case no roll, no crit chance, sorry.

  6. Limit the modifiers that aren’t Complications. I suggest: height advantage (+2), smoke/obscurity (-2), and point-blank and long ranges (+2 and -2, with specific distances depending on the weapon and some weapons having minimum ranges – that’s a design call which is really about how much you want to encourage melee)… and maybe some status modifiers, like disoriented or blinded. XCOM also has an “outflanking” bonus, which AFAICT applies if you move from before to behind the target in the same round you attack (+2), but I think that’s going way too far.

Note how in this set of rules, all cover is destructible – acting as cover is a special condition for objects in the world: cover means you can attack from behind it but still be shielded by it. If it’s not destructible, then it’s just a solid object in the line of sight and it prevents line-of-sight attacks in either direction. I’m also gonna say cover does not apply to melee: you can always strike around it.

If you really, really want something less than a 10% bonus/penalty, consider having it apply to only half the situations, instead of trying to make it 5% and breaking the D10 standard. So, for instance, above we had a +5% scope on a gun. That could, instead, cancel 1 point of the range penalty for long range. It’s a measurable advantage for the weapon but it only affects, say, half the times the weapon gets used.

Obviously I’ve forgotten things. Obviously I’ve promised a load more blog posts. That’s what comments are for.

* what I dismiss here as “the noise of table interaction” is actually what I consider to be the point of TTRPGs. But it’s noise as far as this rules discussion is concerned.

  1. kelvingreen
    February 8, 2023 at 11:21 am

    All of us of a certain lineage and vintage know that UFO/X-COM/XCOM is just Laser Squad with fancy graphics and added Scotsmen.

    I have seen it claimed that Laser Squad is an implementation of the tabletop game Laserburn.

    Of course, Laserburn went on to spawn Warhammer 40,000 and here we are.

    Is this useful data? Probably not, but I thought I’d best mention it. Perhaps there’s something to be learned from 40K here.

    • February 8, 2023 at 8:20 pm

      I remember seeing Laserburn but in those days I wasn’t so into tactical combat.
      I should dig out my copy of Snapshot! and see how many of my realizations it already had in 1979

  2. kelvingreen
    February 8, 2023 at 11:28 am

    As for D&D4 I thought of it about two lines into your exploration, so yes, I think it is perhaps worthy of investigation.

    I played it for a year or so and my group’s main problems were:
    – Flat non-progression: a 1st level fighter and a 10th level fighter did all the same things, just rolled more dice doing them. There was none of that D&D levelling up excitement, because levelling up was an illusion.
    – Identikit classes: in an effort to fix the old Wizard Problem, everyone got “powers” so while no class stood out as special, all the classes felt the same.
    – Everything on the character sheet was geared towards combat: yes, there were skills, but non combat interactions were, as written, “roll high so we can move on to the next fight”.

    You’ll note that no issue concerns the combat mechanics themselves. My memory of those were that they worked well, were nicely tactical, and often fun. You’ll see a lot of people complaining that D&D4 is a decent skirmish wargame but a rubbish rpg, but I don’t think that’s necessarily a problem in the context you’re discussing.

    • February 8, 2023 at 8:22 pm

      I really admire XCOM’s leveling up – within its limited confines as a combat-only game.
      But trying to turn D&D into a combat-only game seems insane to me. It’s the bit of D&D I find least interesting. I can understand therefore wanting to fix it, but what about all the interesting parts? It’s like throwing the baby out because now you have perfumed bathwater.

      • kelvingreen
        February 12, 2023 at 7:04 pm

        It’s weird because a big deal was made of the lack of non-combat mechanics in D&D4 at the time, but there’s an argument that something like B/X is just as “deficient” from that perspective.

        I suppose the difference is that that D&D4 puts so much emphasis on combat that it’s difficult to deny that fighting is the intended focus, but I do have some sympathy for those who argue that you don’t need rules for the roleplaying side.

        • February 14, 2023 at 4:33 am

          B/X doesn’t have rules for… so many things. Which I think is one of its strengths. In general I agree that you don’t need or want rules for the improv that happens at the table. My feeling is you really need rules –
          1. to settle disputes that would otherwise be a problem (you’re dead/no I’m not)
          2. to make non-obvious things happen. Pulp Fiction and Breaking Bad are full of outcomes that nobody anticipated – the gun accidentally goes off or the battery is drained or the world throws some other spanner into the dramatic arcs of the characters and now they have to deal with the new thing. That’s what I really want to roll dice for.

          • kelvingreen
            February 16, 2023 at 2:24 pm

            Agreed. D&D4 lacked anything of the second category out of the box, although like a B/X type game, there was nothing in the design stopping you adding it. I doubt that was intended, but it was there.

  1. February 15, 2023 at 2:20 pm
  2. February 15, 2023 at 7:46 pm
  3. March 20, 2023 at 6:04 pm

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