After looking over my old Animorphs books a bit (like I do), I came to a somewhat startling conclusion: The Yeerks were always going to lose the war. In a meta sense, that’s obvious - they were the villains, and even if the series was going to end with deaths and depression and cliffhangers, it wasn’t going to end with the Animorphs losing. But I mean that within the universe of the story, the Yeerks were well on their way to losing the larger conflict with the Andalites even before the Animorps blew up the ground-based pool and stole the Pool Ship. This became most clear to me in the Hork-Bajir Chronicles and Visser.
And now, because the Internet is lawless frontier land where anything can happen, I’ll present my argument to you. It’ll be organized into three main points: 1) that the Yeerk military was relatively small, 2) that the Yeerks were critically short on hosts, and 3) that the Yeerks were hopelessly outgunned. Once we’ve covered all of this, I’ll look at two additional points: 4) why we perceived the Yeerks as such a major threat in spite of their military inferiority, and 5) why this information doesn’t invalidate the Animorphs’ importance. Let’s do it.
Part I: The Yeerks had a relatively small fighting force
Despite how ubiquitous and unstoppable the Yeerk threat seems in Animorphs, the actual Yeerk military is pretty small. And, as the series reveals, they had been from the very beginning.
In the Hork Bajir Chronicles (HBC from here on), we see the initial uprising that allows the Yeerks to enter space and begin their path of conquest. A small force of armed Gedd Controllers commandeer four Andalite fighters and two heavy transports. Before leaving the planet, they touch down near several major Yeerk pools (I’m gonna go with Hett Simplat and Culat Hesh, since those are the only ones mentioned by Yeerks other than Sulp Niaar, which was right next to the Andalite base and, thus, not a place they were going to make a pit stop). They collect approximately a quarter million Yeerks (and, one would assume, a lot more Gedds/Gedd Controllers, since we run into so many of them later on - though I think we can also assume that they were breeding the Gedds) and then blast off, in search of more hosts.
The Andalites, pretty much immediately, blockade the Yeerk homeworld. So it’s not like a lot more Yeerks were able to escape to join the imperial forces in space. There were certainly some holes in the security cordon, of course. Take the Council of Thirteen as an example. In HBC, it’s implied pretty strongly that they’re still on the homeworld (Seerow’s sure that they must not have known about the insurgency, and he’s never contradicted by even so much as a <Uh, yeah, they were totally there too.> Although if someone said it, it would have been Alloran, by the way. He was famously sassy before that whole quantum virus thing). #54 mentions the Yeerks on Earth receiving a transmission from the Yeerk homeworld to keep fighting, which further implies that the Yeerk leadership is still there.
However, when we actually see the Council of Thirteen (in Visser), they’re all in hosts not indigenous to the Yeerk homeworld (Hork-Bajir, Taxxon, and some unidentifiable), and Visser Three notes that after hearing Visser One’s stories, they’ll probably want a shipment of human Controllers for their own use. So either they got off-planet sometime after the HBC, or they’re able to receive care packages on the homeworld (hopefully with cute little hand-written notes on stationery from the various planets the Yeerks have conquered). We know from an undeveloped story seed that the Yeerks had infiltrated the Andalite homeworld, as well, so they may have had some contacts who allowed them to slip back and forth between the blockade.
Regardless, it still stands that as late in the story as Visser (which was released concurrently with #35), the Yeerk homeworld is still blockaded, and its liberation is a major goal of the Yeerks (which they really hope will be aided by the successful invasion of a Class Five species — WHICH WE’LL GET TO IN A SECOND, DON’T RUSH ME). So that initial escape in HBC was the largest number of Yeerks to leave the homeworld. Granted, every time Yeerks reproduce they create hundreds of offspring, so their numbers would have grown pretty quickly, but that leads us to our second point…
Part II: The Yeerks were critically short on hosts
First, a word on the Yeerk classification system for hosts. There are five distinct “classes” of hosts, which can be described as follows:
Class One: Physically unfit for infestation (i.e. Hawjabrans, Skrit Na)
Class Two: Capable of infestation, but suffer from severe drawbacks (i.e. Gedds, Taxxons)
Class Three: Physically fit for infestation, but are few in number and can’t be bred quickly (i.e. Hork-Bajir, Ongachics)
Class Four: Perfectly fit for infestation, but are too great a challenge to infest (i.e. Andalites)
Class Five: Physically fit, large numbers, breed quickly, and can’t put up much of a fight (i.e. humans)
Host shortages are a constant issue throughout HBC. Initially, Yeerks are only allowed 15 minutes in a Gedd host for training, because of the overwhelming shortage of available hosts. Their encounter with the Ongachics provides them with a small number of new hosts, and their encounter with the Hawjabrans with none. So they’re understandably STOKED(!!!) when they get to the Hork-Bajir homeworld and find a bunch of big, strong, pretty dumb hosts, ready to be taken. They do get a bunch of them, but of course get screwed by the release of the quantum virus, which wipes out a large percentage of the Hork-Bajir population (again, we can assume that the Yeerks subsequently bred them, despite the fact that they’re Class Three — even if they can’t be bred quickly, their physical maturation is pretty quick once born, with Toby as a good example).
Their next biggest host population are the Taxxons, who count as a Class Two species because of their insatiable hunger. And then we hear about a number of other (never seen) races, including the Nahara, the Ssstram, and the Mak. We have to assume that they are either Class Two or Class Three, since neither of them seem to be swelling the Yeerk ranks all that much.
That’s why so much hope was placed on Earth, the only Class Five species they’d located. Conquering the human race would have provided seven billion hosts who were versatile and somewhat user-friendly. This certainly would have given the Yeerks an enormous numbers boost, which would be critical for mounting a land invasion of the Andalite homeworld. BUT… they still would have lost. Because…
Part III: The Yeerks were always outgunned
This comes up again and again in the series. The Yeerks know they’ll never survive a head-to-head battle with the Andalites. Any naval victory we see the Yeerks gain is usually from trickery (i.e. the ambush and destruction of the Dome Ships that came to Earth). In HBC, and throughout, we see that Yeerk tech is largely just recycled or imitation Andalite technology, patched together with stuff they’ve taken from other species (i.e. the Dracon beam, an amalgam of Andalite and Ongachic technology). It’s established that a couple of Dome Ships in orbit can turn a planet into a cinder in a matter of hours.
Yeerk technology, on the other hand, isn’t nearly as impressive. The Pool Ship can kinda look scary, I guess. There’s also the Nova-class Empire Ship, which is presumably bigger than a Pool Ship. However, we only ever see one and have no sense of its armament). You’d think that if it could beat a contingent of Dome Ships, the Yeerks would have used it more.
Now, we know from HBC that the Yeerks do set up shipyards to expand their fleet on conquered worlds (at least they did on the Hork-Bajir world, which is where they created both the Blade Ship and the Bug Fighter). But then we come back around to the problem of manpower, and the fact that there simply aren’t enough hosts to fly the ships even if they built enough to challenge the Andalites. We can also make an educated guess that the Yeerks use most of their fleet to hold onto conquered planets, which is why the Andalites don’t bother trying to liberate them (although another reason for this will be discussed under Part IV).
A case study of the Yeerks’ military inferiority relative to the Andalites can be found in the Anati gambit. For those of you who didn’t spend a weekend crouched in your closets reading decade-old books, the Anati gambit was the Yeerks’ last-ditch effort to stay alive in the fight. Visser One was sent to the Anati system to convince the Andalites that the inhabitable worlds there were their main priority, which would trick them into sending their fleet there instead of to Earth. The Anati system was riddled with small moons, which the Yeerks planned to equip with Dracon cannons, intending to give the Andalites an unpleasant surprise when they emerged from Z-Space. They know that if the Andalites go to Earth, the Yeerks are going to get their asses kicked, they’ll lose their Class Five species and, consequently, the war.
Unfortunately for them, that’s exactly what happens. But even if the Anati gambit had worked, and the Yeerks had somehow infested the entire human population in a matter of months (the logistics of which are mind boggling), they’d still be hopelessly outgunned. Not in a land battle, maybe. But Earth wasn’t going to provide them with more ships, and even with their shipyards working on overtime we’ve established that the Yeerk navy couldn’t stand against the Andalites. Unless those Dracon cannons in the Anati system somehow wiped out over half of the entire Andalite navy, the Yeerk fleet would most likely be defeated before they could ever make landfall on the Andalite world.
So, if the Yeerks were pretty much guaranteed to lose the war, why did they appear to be such a threat? Let’s move on to…
Part IV: Rashomon
It’s really a matter of perspective. Imagine the war as existing on various tiers. The Animorphs are on the ground tier, in the trenches, alone and surrounded by the enemy. To them, the Yeerks seem pretty much invincible. From a higher tier, however — say, from the perspective of the Council of Thirteen or the Andalite High Command — things could look pretty different.
I won’t disagree that the Yeerks ran circles around Andalites for a while. But this was, I believe, an issue of unfamiliarity rather than the Yeerks having an advantage. For one thing, the Andalites had never fought a war on this scale before (or so we can assume, since it never comes up — before this, in the modern age anyway, it seems they were mostly just involved in police actions). They have a standing navy, but even that is relatively small (albeit bigger than the Yeerks’). Elfangor mentions in The Andalite Chronicles that typically the state limits Andalite families to two children, but that there’s been discussion of expanding the limit in order to fill the ranks. And as I discussed before, the Yeerks never engaged the Andalites in head-to-head conflict, preferring guerrilla tactics instead. The Andalites had to learn how to counteract these tactics, and it forced them to fight the war on multiple stages, spread out across the galaxy. Adding to the sense of always being a step behind is that the Andalites seem to focus on preventing further Yeerk advancement. They commit their fleet to preventing new conquests (i.e. Leera, the Anati system) instead of liberating conquered worlds/low priority targets. Hence why they couldn’t just send the majority of their forces straight to Earth on Ax’s word (and when they do decide to intervene, their original plan was basically a controlled burn).
The Andalites aren’t used to being challenged. Their fear of the Yeerks probably has more to do with that (<If they’ve kept fighting us, the mighty and powerful Andalites, for this long, who KNOWS what else they’re capable of?!>) than anything else. But even if they legitimately think that the Yeerks could win the war through anything other than trickery (which they don’t), the Yeerks know that they can’t. Visser, which lets us peek into the upper echelons of the Yeerk military, shows us how desperate the situation really is from their perspective.
There’s another reason why the Andalites might be interested in presenting the Yeerks as an unstoppable contagion to their own soldiers and the rest of the galaxy, by the way: propaganda. We know it exists — Aftran mentions how the Andalites paint Yeerks as a monolithic evil in #19. And it makes sense: the Andalites are, naturally, a pretty peaceful people, so the High Command would need to really up the stakes to get massive buy-in for this war. A nigh-unstoppable evil spreading through the galaxy like a plague would fit that bill perfectly. They’d also want to discourage other species from collaborating with the Yeerks (and counteract some anti-Andalite sentiment that may have existed before the war — more on that below).
Of course, most of what we hear of Andalite propaganda isn’t too far off the mark, even if it does lack the nuance of reality. The Yeerks are on a galaxy-wide mission of conquest (even if dedication to this cause varies among your average Yeerk-on-the-street). However, we also hear some horror stories about the Yeerks that are countered by evidence. In The Andalite Chronicles, Elfangor claims that there are only three sentient species in the galaxy who haven’t been conquered by the Yeerks , and that of those three only the Andalites have the power to oppose them. Over the course of the series, however, we learn about at least five species (the Skrit Na, Leerans, Helmacrons, Hawjabrans, and the Kelbrid) that the Andalites are aware of who are never conquered by the Yeerks. Granted, only the Leerans are possible targets (I’m assuming the Kelbrid are a Class Four species), but still, the numbers don’t work. Also, there’s the claim that Yeerks terra-form conquered worlds (in #7, I believe) to make them more similar to their homeworld. This claim is contradicted by post-conquest visits to the Hork-Bajir and Taxxon worlds.
But the veracity of Andalite propaganda is less important than the primary reason behind it. And this is where this post gets even crazier by slipping into conspiracy territory: The Andalites wanted to paint the Yeerks as an unstoppable force that only they could stop because it would basically validate everything they were doing already.
I used the phrase “police action” above, and that was intentional. The series presents the Andalites as a sort of galactic police force, not only fighting the Yeerks but detaining and arresting Skrit-Na raiders and Norshk pirates. As far as we can tell, however, the Andalites don’t answer to any authority other than themselves. So they’ve basically elected themselves the peacekeepers of the galaxy, which is usually something people only do when they want to maintain control. It bears mentioning that first contact between the Andalites and Yeerks happened because the Andalites came to their planet and immediately established a military garrison (also that the Andalites were pretty racist towards the Yeerks even before they became enemies).
You could imagine that maybe most sentient races in the galaxy aren’t always enormous fans of the Andalites. But when they’re the only thing standing between you the invasion, infestation, and brutal terra-forming of your homeworld, then you might start looking at them differently.
If this was intentional on the Andalites’ part, it works. By the end of the Yeerk War, they’re in a better position than ever to police the galaxy.
Part V: The Yeerks still could have conquered Earth
The Yeerks weren’t going to win the war… but they totally could have won the battle. While we humans have some real moxy (which is an actual point made in our favor by humans, Andalites, and Yeerks alike throughout the series… not with the actual word moxy, but you know…), the Yeerks had us beat technologically.
All of this to say that, regardless of anything I’ve written here, the Animorphs’ fight did make a difference. The Yeerks could have conquered Earth, and if they had, it’s more likely than not that the Andalites would have incinerated the planet from orbit. Instead, the Yeerks lost their final conflict with the Animorphs, their chance at conquering a Class Five species and, consequently, the war.
Of course, if the book Yeerks had developed those Kandrona-free Super Yeerks from the TV show, this might have been a different story.
 To be fair, however, Hett Simplat or Culat Hesh could have been pools established on conquered worlds, which I’m assuming they would have named (despite the fact that the pool on Earth is only ever called “The Yeerk Pool”).
 This would imply that only Yeerks from the original uprising are from the Sulp Niaar pool. That being said, it’s also possible that Yeerks claiming that designation (like Temrash 114, who doesn’t seem like a veteran) are just descended from tripartite Sulp Niaar parents and carry on the pool designation as a matter of tradition/pride.
 Relatively undeveloped, anyway. Hints are dropped in #4 (when Visser Three morphs a creature from one of the Andalite world’s moons) and #18 (Visser Three morphs a kafit bird, and we learn that Samilin-Corrath-Gahar is collaborating with the Yeerks). This was apparently a story seed that was supposed to grow further, but it was eventually lost in the shuffle (K.A. Applegate said this in a Q&A event a few years back, but I can’t find the link). As a side note, the fact that Visser Three’s kafit bird morph is taken by Ax as evidence that the Yeerks may have infiltrated the Andalite homeworld is considered a KASU (K.A. Screws Up) because all arisths acquire a kafit bird morph for training purposes. However, since Alloran was already a full-fledged war prince by the time the morphing technology became widely used, it’s possible that his training process was different from the standardized one that Elfangor, Ax, etc. went through.
 A related point: in Visser, Visser Three implies that the human population is greater than the entire Yeerk military. He says this during an attempt to trick Visser One into committing treason, telling her that if they controlled Earth they could easily wipe out the rest of the Empire and start their own regime. Granted, he’s lying, but it’s a lie that Visser One was supposed to believe, so there was some kernel of truth. When she calls him out on it, by the way, the only thing about his story she takes issue with is the idea that he would ever suggest they work together. This suggests that, even three decades after the initial Yeerk uprising, they still have a critical host shortage.
 For all of their negative qualities, by the way, the Skrit Na were engaging in interstellar travel while the Andalites were still talking with sign language because they hadn’t figured out how to use their own natural ability of thought speak. THEIR OWN NATURAL ABILITY, GUYS.
 I think it’s three, anyway. I don’t have the books on me at the moment, so if anyone wants to fact check this, feel free.
 To be fair, there is one major point that refutes this argument: the fact that the Andalites didn’t immediately use evidence that the Blade Ship had entered Kelbrid space (and Ax’s subsequent capture) as cause to begin making incursions into Kelbrid space. However, this could also be because the Andalite’s didn’t like their chances of open war with the Kelbrid when they’re still recovering from the last war (we also know that the Kelbrid have an enormous amount of territory, and presumably have some serious military might in order to have gotten the Andalites to agree to a we-leave-each-other-alone treaty in the first place).
Note: I didn’t write this, it’s just a long post I wanted to archive here.