The military situation on the Imperial-Federation front was not good as of early 2583. Although the Sidh forces were able to repel any Federation offensives thus far, neither were they capable of undertaking any further offensive actions of their own, as their forces and supply lines were already dangerously overextended as it were.
Pnakotas was a recently-conquered urbanized agri-world of some strategic significance. Captured in 2581, Pnakotas had been taken in a relatively unmolested state, it's industries and extensive farmlands providing a significant boost to the supply of the local frontline sector. Furthermore, having suitable orbital facilities and located just two jumps away from the frontline, Pnakotas made an excellent fleet anchorage and staging ground for reinforcements headed to the frontline. For this reason, preserving Pnakotas under firm Imperial control was imperative to Sidh success in the sector.
Since Pnakotas had been taken with minimal resistence and the human population generally remained passive, the Sidh occupation authorities, while typically heavy-handed, generally refrained from overt repression and other excesses frequently seen elsewhere. Overall, the human residents were being treated firmly but fairly. For this reason, most of the population chose to remain passive in spite of the existence of an underground resistence movement, having heard all too much of what happens to populations who have tried the Sidh patience one time too many.
It is unknown who or why first spread rumors of a Federation liberation fleet having penetrated Sidh defenses and heading towards Pnakotas, but such rumors certainly began to circulate increasingly in early 2583. Since the planet was only lightly garrisoned in light of the need for reinforcements on the front and the relative cooperativeness of the human population, Federation intelligence had relatively little difficulty in infiltrating agents in the general populace and making contact with the resistence. The abundance of brand-new untraceable arms recovered by Sidh forces later during the action certainly seems to confirm that Federation agents had built an extensive and well-supplied network right under the very eyes of the occupation authorities.
In any case, these rumors served to foment popular unrest, mass protests and acts of civil disobedience becoming more and more frequent. The authorities paid them little heed at first, but as the situation began to deteriorate further, proceeded to brutally crack down on any displays of disobedience. At this point, however, it only served to rile up the population further, the small and lightly armed cohort-sized garrison of second-rate troops mostly concentrated in bases near the world's major cities soon finding itself surrounded and overwhelmed, mobs of surprisingly well-armed civilians led by highly-trained insurgents storming Sidh compounds. Cohortarch Mordian Sibellus, the military governor of Pnakotas, would send out increasingly desperate distress calls before his eventual capture. The surviving Sidh troops on the planet retreated into mountainous regions and fortified to the best of their ability in hopes to hold out until reinforcements arrived.
Sidh historians often take note of how well-organized and executed the initial uprising was. In a large part, the success was enabled by the efforts of Theodor Marik, a retired Federation general who had become the de-facto leader of the resistence movement after the Sidh occupation. Although initially unable to do much with their limited resources and popular support, the resistence began to grow in strength after being contacted by Federation intelligence already shortly after occupation. Since the Sidhae were preoccupied with the war and had left the humans of Pnakotas largely to their own devices, the occupation authorities did little to detect or disrupt the frequent shipments of contraband aid to the resistence. Among weapons, ammunition and other supplies, the Federation also sent agents - advisors who would assist the resistence commanders and train their troops for the coming uprising. Two such advisors were Min Jun Yi and Wladek Jankowski, FEMIS agents who would later become prominent resistence commanders in their own right.
It appears that the Federation's intended plan was to distract Sidh forces with a well-organized and massive popular uprising, drawing away precious reinforcements to suppress the revolt threatening their rearguard, and then launch an offensive in the weakened sector, penetrating Sidh defenses and actually accomplishing the liberation of Pnakotas. This did not work out, however, due to several factors, most importantly because of an offensive the Sidhae were planning in the neighboring sector for the next year. In order to regain strength and prepare for it, several elite legions were unexpected taken off from frontline duty out of turn in order to rest and recover to full strength. The resistence also failed in their objective to swiftly secure all off-world communications arrays, allowing the beleaguered Sidh garrison on Pnakotas to send out repeated distress calls and keep the superiors updated about the deteriorating situation. Lastly, the resistence commanders' own failure to rein in their men contributed to the following disaster.
The survivors from the overrun Sidh garrisons were generally captured alive to be interrogated for intel and used as hostages when the Imperial reinforcements eventually arrived. However, asides from grossly underestimating the Sidh willingness to sacrifice their own, the resistence commanders failed to account for the poor discipline in some segments of their troops. While the core of the resistence were without doubt well-trained and disciplined fighters, same cannot be said for large segments of the popular resistence that made up the bulk of the uprising. As the excesses of Sidh troops in their attempts to contain the uprising became common knowledge, and more atrocity propaganda from the Federation reached the people, many would call for blood. It is uncertain to what degree the commanders resisted these calls, if at all, but the end result was the same - the Sidh prisoners were massacred almost to a man, oftentimes in circumstances of shocking barbarity. These atrocities contributed greatly to fuel Sidh own barbarity when the time of retaliation eventually came.
Battle for Pnakotas
Among the elite Imperial legions taken off frontline duty were the Delta Legion under strategos Siberius Keller and the notorious Alpha Legion under archistrategos Arcadius Drax. The troops, weary from almost 5 years of incessant fighting, were looking forward to this unexpected leave eagerly. However, things changed when their legions were called up for an emergency deployment to Pnakotas in full force to crush a human rebellion. Although Imperial Command could have sent any of a number of other legions for the job, it was decided that two elite legions would do the job best, suppressing the revolt quickly and with minimal collateral damage to the vital industries and farmlands. Owing to their proximity to the incident location, the choice fell upon Alpha and Delta legions. Although especially the Delta Legion was significantly understrength, much of it still in the process of gradual rotation away from the front (the rotation deliberately being taken slowly and gradually to avoid suspicion from the Fed intelligence), the present elements were immediately deployed to Pnakotas.
Although the resistence was well-armed and expected a major fight eventually, they certainly weren't prepared for the arrival of two elite legions, as the duty of suppressing rebellions in the rear was normally assigned to second-rate troops and Domestic Security. The Imperium's elite legions, on the other hand, were experts of rapid deployment and urban warfare, seizing the orbital facilities, the space elevators and spaceports with speed and efficiency that took the resistence fighters by complete surprise when they arrived in force in the early morning of April 23, 2583. Having secured these beacheads in a matter of hours, the Sidh forces in orbit could now begin to disembark the bulk of their forces largely unimpeded.
The Sidh troops especially of the Alpha Legion were already infamous for their savage and indiscriminate terror tactics, but now their violence was further compounded by the general irritation of having their long-awaited leave suddenly called off because of some insolent human rebels. Once they learned of the massacre and atrocities perpetrated against Sidh prisoners from the planet's former garrison, that anger exploded into an outright genocidal rage. That said, Sidh troops still exercised restraint for now, as the orders were to suppres the uprising and retake Pnakotas without unnecessary collateral damage to the infrastructure or the workforce.
The following weeks of fighting were brutal, waged mainly in the close quarters of urban areas. Despite being well-equipped and having skilled commanders, the rebels stood no chance against the Imperium's battle-hardened elites. In order to spare the infrastructure, the Sidhae frequently employed chemical weapons to clear their advance, mopping up the survivors. Expecting to be given no quarter, the resistence fighters adopted a no-prisoners policy - which ironically sealed their fate, as the sector's archon-militant was initially suggesting to grant the rank-and-file rebels a conditional pardon in return for their surrender. Executions of Sidh prisoners, however, now made it all but impossible as the final decision about the treatment of prisoners rested with the military, and Sidh commanders who were already disinclined to show mercy to the rebels were now completely disincentivized as their own men were being summarily executed. Although seemingly foolish on part of the resistence, it is worth remembering that the rebel commanders often had only a limited degree of control over their men, the anti-Sidh sentiments were very strong in the population, and the rebels, most of whom had no previous military training, were often panicked and acting irrationally as their situation deteriorated so rapidly.
The Purge of Pnakotas
Incensed about the treatment of the captives from the previous garrison and current prisoners, Arcadius Drax who held the supreme command of the operation demanded that his forces be allowed to make an example out of Pnakotas. His argument was that if spared, the human rebels would merely interpret the Sidh mercy as a sign of weakness, emboldening them for repeated future uprisings and making Pnakotas a "a festering boil in the Imperium's most vulnerable side". However, slaughtering an entire planet's worth of potential converts and labourers wasn't a decision to be made lightly, even Drax lacking the authority to make such call single-handedly.
Since such a scenario would effectively negate the planet's strategic value, and the word of Drax commanded significant authority in Imperial Command, a heated discussion arose. Supporters of Drax's demand to purge the rebellious humans argued that it would be much cheaper and easier to eradicate the troublesome population and replace it with Sidh settlers than to continuously maintain a large garrison that would keep suppressed a hostile population unlikely to cooperate or be productive in the future. Detractors in turn argued that such a purge, were it to become public knowledge, would tremendously fuel the Federation propaganda effort and quite possibly reinforce their war effort enough to make any further offensive actions impractical in the foreseeable future. Interestingly, no side ever argued about the humanitarian aspect of such a holocaust, although there were Sidh leaders who found the idea distasteful for more than just pragmatic reasons.
Eventually, the pro-purge side won out with genocide as the more cost-effective solution and Imperial Command formally authorized Drax's plan with the tacit approval of the Empress.
In the following months, Alpha and Delta legions embarked on a systematic campaign of genocidal extermination on Pnakotas. Where similar actions had occured on a number of occasions before during the Reconquest Wars, most of them had occurred spontaneously, as a result of troops being granted free rein in treating enemy civilians however they deemed fit. Purge of Pnakotas differed in the organization and professionality with which the genocide was carried out, mass extermination of sapient creatures being carried out on an industrial scale with a truly-devious efficiency.
Contrary to most other genocides perpetrated by the Sidhae, there was little in the way of arbitrary abuses, torture and degradation on Pnakotas - there simply was no time for that, as the military was faced with the monumental task of wiping out 250 million people as quickly and efficiently as possible under wartime conditions. Consequently, the process was maximally streamlined. First, populated areas were shelled with chemical and biological weapons to flush out any residents that weren't immediately killed. The chemical agents were deliberately chosen to be short-lasting to simplify decontamination efforts, while the biological weapons used were chosen for a long incubation period and potency, to ensure that any survivors of the initial attack who would escape the following roundup would eventually die in any case and infect any other humans they might come into contact with. Troops would sweep through the targeted areas to make sure no survivors with potential NBC protection remained in hiding, and automated decontamination teams would then be sent in, drones collecting the bodies for recycling.
After being flushed out from residential areas and hideouts, the survivors would, depending on their level of organization and resistence, either be killed on sight or rounded up into ad-hoc concetration camps. Prisoners of these camps would then be herded into "death factories" - truly devious contraptions trumping even the efficiency of Nazi death camps in the 20th century. Unlike the Nazis, who would often subject their prisoners to torture and degradation before their eventual execution, the Sidhae would not bother with such inefficient trivialities. To minimize the demoralizing effects of participating in such massacres to the troops, the death factories were almost entirely automated, their only living staff being the guards securing the perimeter against escapes, and a handful of technicians supervising and maintaining the equipment. Mechanical moving walls would force the prisoners out of the arriving transports and in the desired direction, dividing them into tightly-packed groups, which were then pushed into rooms with massive industrial meat grinders normally used in knackeries under retracting floors. Despite the evident horror of being ground up alive, the victims would only suffer for a few seconds at most. Their ground-up remains would spill on conveyor belts underneath and be fed into plasma furnaces to be atomized into their basic constituent elements, the flammable gases being used to power the furnaces and the factory itself, and the rest being condensed and reclaimed for later reuse in industry. To speed up the process, some death factories did away with the grinders entirely, instead feeding the victims directly into the plasma furnaces, a fate arguably less cruel than being ground up, as the temperatures in the furnace would incinerate any victim almost instantly. However, such method was determined to actually be less efficient, since whole bodies took more time to incinerate than a steady stream of human slurry, and the prisoners would further tend to struggle and agglomerate together in efforts to escape their doom, sometimes clogging and extinguishing the furnaces altogether and stopping the operation of the plants until they could be cleaned and reignited. Such setbacks in turn meant that the still-living prisoners had time to think, organize and attempt to escape, or at least die trying, further hampering the site's efficiency. Consequently, the no-grinder facilities were done away with and returned to the established grinder-and-furnace standard.
By late September when Pnakotas was finally declared pacified, less than 5% of the original population survived, with most remaining in hiding. To minimize their chances of survival, the Sidhae systematically harvested all available food sources and destroyed the rest. Sidh settlers were in the process of being brought in already since July to man the most vital industries and replace the extirpated population.
To this day, historians still debate hotly whether the Purge could have been avoided. Those opposed to the purge say that the last vestiges of organized resistence had been wiped out already by mid-June, with all of the resistence commanders killed in action, thereby making further extermination unnecessary. Furthermore, as one of the last Federation intelligence ships to leave the doomed planet managed to ship out evidence of genocide against the best efforts of Sidh authorities to contain information of it, the Purge of Pnakotas served to incense the Federation into action, massively boosting their enlistment rates and increasing their military production as many workers would become willing to work for extra hours without pay to help the military avenge this atrocity. This ultimately justified the concerns of those opposed to the genocide, for while the Federation ultimately proved unable to push the Imperium back even with their fresh reinforcements and motivation, neither could the overextended Sidh forces hope to conduct any further offensives in the predictable future.
While to humans Pnakotas stands a testament to Sidh inhumanity and irreconcilable enmity with Mankind, Sidh own views on the matter are more divided. Arcadius Drax, the architect and director of this genocide, has remained adamant about its necessity to present day - in his words, it is an action that no Sidh should take pride in, but one that was necessary to carry out nonetheless, as the price of failure could have been much greater. His main accomplice Siberius Keller was less outspoken on the matter until his death in 2617, stating that some things best be left buried in the past where they belong. The Empress has never made an official statement concerning the Purge of Pnakotas, neither confirming nor denying her involvement or approval of the action.