War, in real life, is waged by living things - most often humans. Usually, there is a winner and a loser. These are clearly established concepts: some people live, and some people die. For every kill gained on one side, there is a death on the other. This is the basis behind "PvP", or player versus player, combat. Sometimes, things that should be depicted as fair and equal "PvP" combat is broken down into PvE (player versus environment) combat. This is combat where, instead of intelligent, complicated humans, the enemies are simple, stupid, and perfectly okay to mow down by the thousands.
This is something that I've talked about before, with a bent towards inter-human conflict twisted towards a non-human perception (i.e. "you're killing humans but they're dumb and evil"). Today, I want to talk about something different - "armies" versus "hordes", or essentially "PvP/PvE on a larger, setting-mandated scale". This refers to a difference in perception between entire groups, and justifying the nature of either intelligent or unintelligent tactics. It also focuses on concepts of sympathy, and whether or not the member of the larger group is truly an "individual".
The major thing about "armies" is that they are composed of individuals. It doesn't matter whether these individuals are soldiers, mercenaries, militia, or civilian, the concept of an "army" in this particular sense is referring to groups made up of human beings. Therefore, armies are sympathetic, because they are populated by sentient beings. A soldier in an army was born, raised as a child, and grew up into an adult. They developed skills and traits over their lives. Even if these traits are barely (or not) touched upon, they can be extrapolated. An army may be an enemy, but every death in an army is the death of a sentient being; another mother's son senselessly slaughtered, another father who'll never come home.
Another important note is that armies are numbered and limited. Every death in an army is one less soldier to fight. Armies are not infinite, and they have to deal with things like supply limitations and monetary costs. If the recruitment, training, and equipping of a military unit is shown in detail, it's an army. Armies exist as part of a logical, rational chain, whether it's training soldiers (from a limited population) or providing equipment and gear (with finite production). The key word is limitations: every death in an army matters, and (at some level) exact numbers can be pulled up to show casualty percentages and current strength. Any combat unit with limited numbers and supplies is an Army. Hence, Armies are found in real life and Hordes are not.
To sum up:
1) Armies are sympathetic in an individual level, because they're regular folks just like you and me. They were kids once, they have or had families, and now they're part of a larger militaristic whole.
2) Armies can be depleted. Every death matters, at the very least because now there's one less person to fight. Replacing the dead involves recruiting new people, which may not always be feasible.
3) An army runs on an actual chain of supply. They can be starved, or made bereft weapons or armor. They are a machine that requires constant fuel and attention, and without it they will fall apart.
Hordes are numerous, if not numberless. Hordes are comprised of creatures who barely even register as sentient or sympathetic - they may as well spring up fully born out of holes in the ground (and some do). The goal of a horde is to be an antagonist, usually a perpetual one. Members of a Horde are little more than teeth-gnashing monsters, even if they're "human", and obtain weapons and armor from a vague source (often improvised from scrap metal). They are generally suggested to pillage and plunder to account for their supplies. If defeated, they slink off to their holes and re-emerge some time later with replenished numbers. Their members are irrelevant, because they exist (both in the meta-sense and in-universe) to fight and die.
Hordes are the ever-nipping teeth on the heels of civilization. They are not numbered, and they are rarely sympathetic. They can be turned back for a time, but do not operate on the same laws of resupply and recruitment that armies do - and hence, before long, they will be back. Hordes are like waves crashing against rocks - they might not make a huge impact, but there's enough of them that they're effectively infinite versus to a finite army, and thus can wear them down through attrition. Their members are unimportant, and if they receive any background it will generally be along the lines of "fighting and killing".
Of course, Hordes aren't limited to the traditional mob of orcs or barbarians. Any enemy that (a) is never shown to worry about supplies and (b) is effectively infinite and unsympathetic falls under the general purview of a Horde. This means that, for example, zombie scenarios are about Hordes, usually because of the imbalanced threat they present to PCs and NPCs (PCs easily mow through thousands of zombies, NPCs are swarmed and eaten immediately to reinforce the undead population). Numberless bugs like those found in Starship Troopers (the movie) or Warhammer 40,000 are also a sort of Horde. Finally, "evil armies" (such as stormtroopers) may also constitute a Horde if they are faceless and characterless - i.e., bereft of humanizing elements, but this status is generally at odds with what the army in question is meant to represent (human beings fighting for an organization).
To sum up:
1) Hordes are unsympathetic because they're not "human beings".
2) Hordes aren't supplied, at least on-screen: it's just sort of implied that they get their materials and reinforcements from "somewhere". Often they will "spawn" members, rather than "recruiting".
3) Hordes are usually an antagonist because they're infinite and unsympathetic, and thus must lack in skill and finesse to be "balanced".
An "army" versus a "horde" is like a SRPG versus an RTS. The former is based on working with limitations - limited weapons, limited equipment, and above all limited soldiers. Armies foster investment and care, because even though death in battle is highly likely, they're still human beings. They're worth more alive than dead because the skills they've learned through their life are hard to replace. Even if units are treated as being disposable, they're shown in a light that suggests they're "real people", rather than abstracted combatants with no backstories or lives of their own. The fun of playing an Army comes from investment in the well-being of the soldiers combined with the preservation of veterans and their valuable experience.
In contrast, when playing as a Horde, units are meant to be massed and thrown headlong at foes. The enjoyment comes not from caring about individuals but about seeing giant groups overwhelm their foes. Yes, many may be lost, but they are unimportant - and hence, instead of a heart-tugging battle for survival, it's more about the player having fun winning battles. In a lot of cases games that are meant to be treated like "armies" are viewed in a more "horde-like" way, but in general it comes down to whether or not you can relate to (and care about) individual soldiers. If you can, and you're motivated to keep them alive, then it's an Army. If you can't, and you're just meant to accept that your charge will result in huge numbers of casualties, then it's a Horde.
Army versus Army
When two armies fight, the motivation and intent of the conflict is going to be tempered by the need to keep people alive. Hence, two limited armies fighting each other are going to use whatever tactics and strategies they can out of the need to avoid losing resources. This is the true form of "PvP": Two sides that both have reasons to use tactics and preserve the lives of their soldiers. If resources aren't infinite, the life of a soldier matters more. Why throw them into a meat grinder when they're better used to ambush or distract? In such cases the traditional scenario of a bridge or breach may occur (i.e. a chokepoint where many soldiers will die), but attacking it is an unpleasant necessity, not a matter of course.
Skirmish-level games, such as Mordheim or Necromunda, generally fall into this category if the players care at all about winning. The harder it is to replace the dead, the more motivation players have to keep them alive. That is the basic logical concept here: not of abstract naming concepts, but about whether the player (or commander) is motivated to keep individual soldiers alive, either through veterancy or sympathy. The further out the perspective is, the less sympathetic or important individual soldiers are.
Army versus Horde
This is the classic PvE setup: protagonists versus antagonists, or plucky outnumbered heroes versus faceless masses. On the one side you have humanized, recognizable characters. For them, every death is important, but they can be sure they're taking dozens of their foes with them for each one they lose. People love this sort of thing - from Rorke's Drift to Helm's Deep, it emphasizes the heroics of the Army at the expense of the Horde. The less individuals there are, the less expendable they are, and vice-versa. When a Horde member kills an Army member, it's a huge deal - when it's the other way around, it's just another marker on the kill-count.
Of course, this becomes an issue if both sides are meant to be human. It's one thing to mow down the masses of incoming bugs, but if both sides are supposed to be equally intelligent and capable, then it's a total mismatch. In general, the "superior" side (numerically superior, anyways) has to allow itself to be cut down in droves in order to emphasize the strength of the few, proud soldiers. In reality, having more troops is a huge advantage, not just in direct strength but in the number of maneuvers you can execute and plans you can deploy. It's possible to beat a larger force with cunning and wit, but that's reliant on the other force not also having cunning and wit.
Horde versus Horde
This is probably the most common multiplayer mode: two sides with near-infinite resources throwing troops at each other with little to no regard for preservation of lives. HvH can be the most fun mode because there's no sympathy to slow you down. Both sides consist of fighters with no individual backstory, and the game is just to kill as many of the other guys as you can. It's guilt-free gaming: you can take as many losses as you like and nobody, not even the characters in-universe, really cares. It's intentional sociopathy, sometimes to the point of comedy. It's not Joe Somebody who came from the town of Whatever and had a family and friends and all that, it's a "unit".
One of my favorite examples of HvH combat is Orks versus Tyranids in Dawn of War 2. It's pointless fun - both sides want to kill each other, and neither is concerned about their own well-being. You could say that about every faction, of course, but there's no reason to get attached to either faction in the same way that one might be attached to Space Marines or Imperial Guard. They're limitless and they don't care about dying. Units can still distinguish themselves through veterancy, but it's just that they're better at killing, not that they've earned a rest or something - they don't want a rest, they want to fight more. It's perfect for a game that doesn't treat characters like real people in any case.
So, To Sum Up:
1) If a group is limited, the attention focuses on individuals, and they become more sympathetic to the audience or player.
2) If a group is unlimited, then it's impossible to think of them as human beings, because human beings are not "unlimited".
3) The former rewards investment, the latter rewards immediate action.