Following the Imperium's withdrawal from the Age of War and the Second Pilgrimage, the remnants of the Old Imperium were in shambles. The immense destruction wrought upon the Sidhae by their human and Skargh enemies had left the survivors with a massive collective trauma that set the Sidh race on the dark path of extreme xenophobia and rabid hatred of humanity and aliens characteristic to contemporary Sidhae.
Eager to one day exact a terrible revenge for all the wrongs and atrocities of the Age of War, the surviving Sidhae reinvented their Imperium, building its entire economy, ideology and lifestyle from ground up to support a massive war machine that would one day return and reclaim what was rightfully theirs. Any notions of tolerance or peaceful coexistence were discarded in favour of survival at any cost and ruthless pragmatism. In the following centuries of rebuilding, generations of Sidhae were brought up with the idea that they will one day return and exact revenge on those who almost destroyed their race before.
In the meantime, Mankind and the Skargh were busy slaughtering each other over the scraps of the Old Imperium, eventually grinding each other down into an exhausted stalemate by 2500. At this point, Sidhae were largely forgotten and believed to be extinct or gone off to make a new home in a distant part of the galaxy for good, sporadic sightings of Sidh scout ships and freelance explorers largely being dismissed as spacer tall tales or simply human pirates or mercs using derelict Sidh ships.
Weakened by the centuries of warfare, both rivals had finally backed down, entering an informal cease-fire to rest and rebuild for at least a generation or two. This sudden absence of seemingly-perpetual war would result in growing domestic unrest. Left without the steady source of income that major military contracts would provide, the megacorporations of Mankind would grow restless, at times going as far as clandestinely starting insurgencies on Federation planets just so the government would be forced to contract them for supplying the pacification forces. Corporate espionage and sabotage was also rampant as the megacorps vied for market shares in the suddenly-exploding civil market that was no longer restricted by wartime rationing under conditions of total war, oftentimes devolving into violent skirmishes between corporate security forces, a period of time known as the Corporate Wars. The Skargh too would face significant infighting, the various clans and noble houses being left without an enemy to fight, cutting off the steady stream of plunder that had provided a significant part of their income. They consequently turned upon each other, intrigue and power schemes more often than not giving their way to open violent confrontations. Focused on dealing with domestic rivalries, neither side paid much attention to the dark clouds gathering just a few dozen light-years outside their borders.
These circumstances provided Sidhae with an excellent opportunity to strike. Having infiltrated covert operatives on a number of Fed worlds some years in advance, Sidhae were quite aware of the situation in the Federation, and to a lesser degree also in the Skargh Empire. However, their increasing activity in preparation for an invasion did not escape notice of the Federation intelligence services, which warned the authorities of a potential return of the Sidh menace. These warnings largely fell on deaf ears, the few in the Federal government who were willing to take them seriously limiting their reaction to sending token reinforcements to some worlds believed to be under threat. Their caution can be understood in context of the worlds in question being within the partitioned areas of the old Imperium, close to Skargh space - deploying an adequately sized force would likely alarm the Skargh and carry a risk of reigniting conflict with the Skargh Empire in a very inopportune time.
It is unknown whether the Skargh knew of the Sidh threat and what their reactions were, but given the lack of defensive preparations and generally-light garrisons along the Sidh-Skargh frontline especially in the first year of the offensive, it is highly doubtful that the Skargh took any potential warnings of a Sidh return any more seriously.
The Onslaught of the 50's
Both sides were caught completely unprepared when the Sidhae finally struck in the fall of 2549. Over 50 worlds fell in just the first two weeks of fighting, dozens more following in quick succession. Both humans and the Skargh intially refused to believe that the Sidhae had returned, accusing each other of staging false-flag attacks and engaging each other in several completely pointless battles before finally coming to accept the truth. The scale and pace of the Sidh invasion, unseen since the early days of the Age of War, also meant that communications with invaded worlds were very often lost before distress calls could be sent (more often than not owing to Sidh sleeper agents infiltrated to sabotage defenses years in advance), the garbled and scattered reports making it difficult to assess the whole picture. It was only in early 2550 that the Federation and the Skargh Empire managed to muster the first serious military response.
Although the defenders eventually mobilized their forces effectively enough, the seemingly-inexhaustible tide of Sidhae would gradually but surely press on, crushing their foes with brutal efficiency and meticulousness and using a range of new advanced technologies thereto unseen by either side. Over the coming decade, hundreds of worlds would fall to the Imperium, some after being starved into submission by lengthy sieges, others after a few months of savage fighting, and yet others simply surrendering to spare themselves the Sidh wrath. The Sidh juggernaut was seemingly unstoppable, its foes growing increasingly desperate.
The entirety of the Reconquest Wars was characterized by extreme brutality especially on the Sidh part, entire planetary populations being routinely subjected to extermination or enslavement. Since the action took place almost exclusively on enemy soil, human and Skargh civilians would suffer the most, retaliatory atrocities against Sidh civilians being limited to isolated incidents when a counter-attack managed to overrun a resettled Imperial world. While the human and Skargh forces would no doubt return the favour to any captured Sidh, the suicidal determination of most Sidh soldiers and their general spirit of giving no quarter and asking for none meant that such prisoners were rare and incidental, as opposed to the scores of unfortunate humans and Skargh who would fall under Sidh occupation. The harsh treatment of subjugated populations was briefly relaxed in the middle of the Reconquest Wars only to reignite with a vengeance again in the 2580's, firmly cementing Sidh reputation as brutal and merciless killers in the human memory. Interestingly, the Skargh seem to hold no particular ill-will towards the Sidhae for their extermination and enslavement of Skargh civilians on conquered Skargh worlds, largely because of their habit of doing the exact same things to their enemies and expecting to receive no different.
The stalemate of 2560's
After a decade of high-intensity war, the Imperium's resources were finally stretching to their limits, even though their almost-exclusively war-oriented economy could support considerably greater overall military forces than either of their foes. As the offensive ground to a crawling pace, giving Federation and the Skargh a much-needed respite, the defenders would strike an informal truce to refrain from mutual hostilities while the Sidh threat persisted, although mutual enmity and distrust remained much too strong for an alliance of convenience. While the Sidhae were regrouping and reinforcing their hold on the newly-reclaimed Imperial worlds, both sides would shift their industries and recruitment efforts into overdrive to reinforce the frontlines and even launch the first effective counter-attacks, the Imperium being forced into several retreats for the first time in over 10 years.
Displeased with their lack of progress, the Empress would chastise her strategoi, having several of them replaced. While a large-scale general advance was no longer possible, Sidhae now adopted a new strategy of launching daring raids against strategically-important worlds deep behind the enemy lines. Heavily fortified fortress-worlds were now simply bypassed rather than captured in costly assaults, leaving behind just enough naval forces to maintain a blockade and prevent their reinforcement or evacuation.
During 2560's, neither side made major territorial gains, warfare largely revolving around naval engagements and mutual raids.
The Bloody 70's
The Imperium's offensive regained momentum when Arcadius Drax ascended to the command of the Alpha Legion, one of the Imperium's most elite Army legions, in 2567. Ambitious, innovative, extremely bold and utterly ruthless, Drax was the perfect kind of officer for the job of breaking the stalemate. Utilizing extremely aggressive shock-and-awe tactics that would become the trademark of the Alpha Legion under his command, Drax managed to accomplish more in a few months than the combined force of 120 legions had done in the past five years in his theater of operations. This success fast-tracked him for promotion to Archistrategos, placing him in charge of an entire sector of operations where the Sidh frontline touched with both Fed and Skargh territories. Inspired by his example, numerous other commanders adopted Drax's audacious tactics, once again putting the enemy on a slow but steady retreat.
These bold operations had a heavy cost, however, the Imperial forces suffering more casualties than in the past two decades combined. Despite major territorial gains, eventual reclamation of 95% of the old Imperium and making deep incursions into Fed and Skargh space, Sidh forces were again on the brink of exhaustion again by 2580.
The Great Stalemate
As the second Sidh offensive ground to a halt once again, the Feds and the Skargh would launch a series of counter-attacks, having somewhat adapted to the new Sidh tactics and fielding new innovations of their own. Both sides had independently determined that the success of Draxian tactics depended largely on the shock of an initial assault. If the defenders managed to weather the first two weeks of invasion and grind down the momentum of the initial planetary invasion, the Sidhae would be forced to pause their assault to regroup and rearm, allowing the defenders to be reinforced and counter-attack, more often than not successfully pushing the Sidh forces back. With their resources and supply lines already stretched to the limit, Sidhae would often elect to abandon the invasion altogether rather than risk a protracted war of attrition in which the humans and Skargh had the advantage of numbers and home soil.
The counter-offensives managed to retake significant territories, forcing the Sidhae into shortening the frontlines. It was only through the cunning actions of Archistratega Aurelia Creed that this tactical withdrawal did not become a full-scale retreat with unpredictable consequences. Being a cunning politician as well as a military commander, Creed leaked information to the Federation media about the Skargh accepting human POWs from the Sidhae as ransom payment for Sidh captives and detailing the cruel conditions of their enslavement. Federation public was in uproar, regarding such underhanded deals as a breach of the informal truce between the two powers, public opinion and protests from senior military brass forcing the Federation government into authorizing reprisals against the Skargh. The consequent reignition of hostilities between Mankind and the Skargh took off the pressure from the Sidh forces, allowing them to reinforce the frontline sufficiently to hold against further incursions, although Imperium's resources no longer allowed for any major offensive operations either.
At the same time, numerous insurgencies erupted on occupied Federation worlds as word of Sidh defeats reached the native populace. These uprisings were brutally put down, culminating in the infamous Purge of Pnakotas in 2583, when the entire human population of Pnakotas of over 250 million was exterminated in retaliation for their mass pro-Federation uprising. While this atrocity provided ample fuel for Federation war propaganda and resulted in an attempted Fed reprisal action, this last offensive of the war was thwarted, situation again ending in an exhausted stalemate, neither side able to sustain a three-way war for much longer.
Although no formal peace treaty or cease-fire was ever signed, and all three powers still formally remain at war, no major military action was taken after 2583 with the tacit agreement of commanders from all three sides, at least nothing of the scale seen in previous decades. Frequent raids, violent skirmishes and local incursions spanning a few star systems would still occur, probing the enemy for weaknesses, but all three sides would thereafter refrain from escalating these operations any further. Realizing that her forces could not possibly meet the original strategic objectives of annihilating both rival empires in a single stroke, the Empress contended herself with having met the objective of reclaiming most of the Old Imperium. This relative peace would continue until the Skargh Invasion of Altoris in the 2640's.