Rachni wars

I think the Rachni Wars was as near a full scale war as we could possibly get within the timeframe of the galaxy at that point. Obviously, humanity, the turians and quarians weren't involved but as far as length and impact I would say that it could be seen as the second most devastating war after the Reaper invasion of ME3.

The Krogan Rebellions, while violent, didn't cause as much devastation due to the fact that the Citadel had preemptive sabotage contingencies in place, and that the Krogan were an established nation as opposed to being an outside context alien invasion.


Paradoxically, the Rachni wars probably caused less environmental damage to the worlds that did see fighting compared to the Rebellions for the simple fact that the Rachni weren't a species known to waste resources. Indeed, the only mention of devastation and scoured planets during the Rachni Wars was when the Krogan joined the fighting and proceeded to nuke everything in sight.

So my thoughts here are:

1/ It's called "wars" plural, so my assumption is that it's a scale of many incursions and many struggles.

2/ If you look at the timeline:

• 1 CE the "war" begins when explorers open a mass relay to previously unknown systems
• 80 CE, first contact between the krogan and the salarians (ME3 codex entry on Tuchanka)
• 100 CE, krogans fully uplifted and mobilized
• 300 CE, total extinction declared – 200 years of war for the krogan

Like you said, a war of grinding attrition of the course of centuries. When we are faced with the decision of what to do with the Rachni Queen in ME1, Wrex tells us on Noveria (if you take him with you) that "...millions of my ancestors died..." to defeat the Rachni. The losses must have been staggering even for the krogan, who breed quickly and in great numbers when conditions are right for them.

3/ I also look at the "80 years of war" tidbit. Contextualizing this with the main species of the galaxy:

• For asari, that was less than 1/10 to 1/15 of their known lifespan.
• To salarians, this was two lifetimes, and a minimum of two but likely at least three or four generations. 
• It’s at least a significant chunk of elcor lifetimes (we have an instance of one elcor character being considered quite old at 398 years.) 
• And while we don’t know what pre-Morning War quarian lifespans look like, in the 2180s CE they apparently had a lifespan approximate to human lifespans, which could mean this conflict up until 80 CE likely would have been around 1/2 to 2/3 of their lifetime.

What exactly were they doing during these eighty years of conflict – beyond the obvious, of fighting the rachni in a long, protracted war?

We know they apparently tried negotiations, and these were futile / deemed failures because it was impossible to make contact with the hive queens that guided the rachni troops above the surface of their toxic homeworld and in the wider expanse of space. Of course, we know that the lack of proximity isn’t necessarily the issue, as in Mass Effect 2, if we spare the rachni queen on Noveria in the previous game, we can meet an asari on Illium who has a message from the queen. And so, with that said, there’s evidence that the rachni may not even have been receptive to negotiation even if members of the Citadel species could have reached the queens. When you speak to the trapped queen on Noveria, she talks of the genetic memories carried forward from queen to queen, and of the “...sour yellow note...” “...a tone from space [that] hushed one voice after another...” The meta implication is that the rachni were indoctrinated or otherwise influenced by the Reapers (or potentially even the Leviathan) who we know from additional gameplay and lore both have a history of manufacturing conflict between various species in the Milky Way, including but certainly not limited to, the Zha’til during the previous cycle and the Heretic Geth during the present cycle.

Obviously, their efforts weren't enough, and they turned to the krogan.

4/ The actual lore related to what happened to the rachni worlds:

The Mass Effect fandom wiki has within its Krogan page a single throwaway tidbit of lore that actually has some fascinating implications in terms of the krogan and their relationship with the rest of the galaxy, as well as more pertinently the scope of the Rachni Wars. In talking about the immediate-post Rachni War period, it says:

• "...and were given not only the conquered rachni worlds but other planets in Citadel space to colonise…"

(This isn’t something that can be found in the game files, and it doesn’t appear to have a basis in the Mass Effect novels or comics. In fact, if you look at the meta data on the page, the edit history shows that it came from an administrator named TULLIS who added it in February 2008, meaning it predated Mass Effect 2 and Mass Effect 3, and it technically predates the publication of the second Mass Effect novel. Where it comes from, at the time of writing this paper, is uncertain. The user, Tullis, as it turns out, was an employee at BioWare during that time (and may still be, unknown), but she worked in the Austin division and apparently mostly handled SWTOR, so it’s possible that she may have had more access to internal documentation or inquiries with the BioWare Edmonton team than the average layman might have.)

We know that the Rachni homeworld itself was toxic to begin with, and it is implied that the Rachni would make worlds toxic when they conquered them – which is why the krogan were needed to fight against the rachni in the first place, where they were "reclaiming conquered "Council worlds" in addition to conquering rachni worlds:


We have also have a codex entry that describes some of the devastation visited upon the Rachni by the krogan, specifically the Rachni homeworld of Suen, and specifically with WMDs:

* ~ * Surface bombardment + ground troops to personally deliver WMDs to the heart of the queens' lairs. * ~ *

A question I've seen crop up before is, "Why didn't the council species just glass the rachni planets?" My guess is that, since we can reasonably infer from what was required of the krogan to attain victory (ie going underground into the warrens), they probably did try glassing planets with surface bombardment from orbit. And it wasn't successful.

A conversation in Mass Effect: Andromeda from Nakmor Drack - someone who fought in the Krogan Rebellions – tells us that even at the time of the Krogan Rebellions in circa 700 CE, the ruins of the Rachni War were still a smoking mess:

• CORA: “I’m guessing even you weren’t around for the Rachni Wars.”
• DRACK: “Before my time, but you could still see the aftermath when I was young. Ruined cities devoured from below, smoke still pouring out of old nests centuries after my people burned them out. And the bleached skulls of the rachni queens in the dust.”

Keep in mind that the timescale here is, the end of the Rachni Wars circa 300 CE, after 200 years of warfare between the rachni and the Citadel-backed krogan, with 400 years characterizing the interwar period to the beginning of the Krogan Rebellions in 700 CE, circa when Drack was a youngster. There’s a couple of inferences you can make there:

• 1. It’s possible Drack saw these smoldering craters and the bleached skulls himself. It certainly lends itself to the suggestion that krogan were living on, or at least accessing, these planets. But this isn’t explicit confirmation, as they could have just seen these sinkholes from space, which is what seems to be implied by the Suen entry. Could go either way.
• 2. What these do confirm is that, not only were these planets very likely already toxic because of the rachni presence (terraforming, perhaps?), but they’d been further ecologically degraded by WMDs that were still radioactive and smoking, 400 or more years after they had been detonated. Double whammy for the planets.

So.

With that said.

How you choose to interpret this is up to you, but I think given the fact that the krogan were involved in 200 years of warfare, and the way they defeated the rachni was by fighting their way to the planets occupied by the rachni queens, delving into their lairs, and detonating local WMDs, indicates that it was an incredibly brutual war of attrition that imo had them systematically retaking planet by planet by planet, until they'd driven the rachni out of council space, back into rachni space, and then ultimately cornering them in their last stand at Suen, the rachni homeworld. If I had to posit a guess, I'd wager that each planetary incursion to eliminate the rachni hold, root them out, and drive them further back, was probably its own campaign. 

The on-the-ground planet-by-planet wars of attrition is one thing. But the other thing I find myself thinking about with this is like... You have to think about the sheer level of violence and destruction that was visited upon every single one of these worlds that the rachni had occupied – that centuries later the nests were still smoldering. And you have to think about the fact that the Citadel species – namely the salarians and the asari – were very, very much okay with this violence (as a means of survival, sure, but nevertheless). They wanted it. They cultivated it. They utilized it. They were more than happy to spend several decades (80 CE uplift to 100 CE deployment of the krogan; half a lifetime for salarians, and probably at least one major generational turnover) helping the krogan uplift and breed to numbers of a horde that could stand against the rachni. And they armed this horde. They manufactured and developed military technology en masse for this horde, and sent it forth to commit great acts of violence. And we know that there were, bare minimum, millions (plural) if not billions of krogan warriors being armed and supported. I have to imagine that this was systems-wide mass-mobilization of industry to develop ships, bombs, armor, guns, and other tools of war.

If we revisit the species lifetimes things, 300 years of war during the Rachni Wars was: 1/5 to 3/10 of known asari lifetimes; ~7.5 salarian lifetimes, and probably 10+ generations; nearly a full elcor lifetime; and two quarian lifetimes, likely 3+ generations. The salarians had known nothing but war with the rachni for generations. Every asari maiden alive at the end of the war had likely known nothing about warfare their entire adult lives, and likely many of the matrons cut their eyeteeth on combat with the rachni. And while it's doubtful that there are any asari alive in ME1-3 that were veterans of the Rachni Wars, like the krogan, the oldest of asari alive almost certainly had in their youth parents or other living relatives who were - a much more direct connection to this ancient history. 

5/ They were so worried about the survival of the Rachni that they put up Listening Post X-19 in Suen's orbit, a post that is still manned even in ME3, nearly 1,900 years later. This listening post is older than the oldest known members of the longest-lived representatives of the Citadel species.

We have a small sense of the scope of krogan lives that were lost fighting the rachni, and of the timescale of the war, and we can pretty reasonably infer the impact it had on the lived experiences of the Citadel space species based on their known lifespans. I think it's safe to say that the Rachni Wars were A Pretty Big Deal that had a profound cultural impact; it's the war that has formed the shape of the galaxy as we know it.