For the operation Shadowstrike prepared to use almost all the Tau armed forces on Tarsis he could spare. The strategic reserve was allocated and the bare minimum left on occupation duties. Some in the Tau camp feared that were the attacks to go wrong, they would find themselves at the mercy of the Imperium.
The Imperial forces were not unaware of the build up for operation Poseiden, but very few in the high command realised quite what the Tau were planning. Not even General Cunningham, with his experience of Melberg, was aware that Shadowstrike was preparing to strike fast and hard, simultaneously against the Imperium and Chaos utilising practially all their strength.
Initial aims were modest, to cut off pockets in the north and south and move against the enemy held cities of Montgate and East Plainbrook. However Shadowstrike and his commanders were under orders to take opportunities where they saw them, in contrast to the dogmatic and autocratic style of the Imperial armies. In this way, the Tau could make best use of their assets, and overcome their apparent material inferiority with respect to the Imperium.
Assault on Chaos
After his successful attack out from Sapphire Harbour Elan Ro was once again mounting up his force into Mantas and relocating. This time he was placed against what remained of the Chaos forces. While he’d been off planet the might of Chaos had crumbled, and Shadowstrike intended to capitalise on this.
What Elan Ro found was an entrenched force of Iron Warriors ready to try and repel his advance. His mechanised units moved forwards and Stealth units in advance of the main force began to harry the defenders. The Iron Warriors were lacking in long range anti-tank weaponry, only as small contingent of Predators, which were suppressed and then systematically annihilated.
The main fighting occurred where the Iron Warriors decided to make a stand, and the Tau advance hinged on removing these elite elements before a mop up of cultists and other irregulars could allow the advance towards the Chaos held city, as envisaged by Shadowstrike, could begin. Only then would Fastblade, his forces in reserve, be able to assault the city.
Elan Ro’s initial advance was cautious, as he had intelligence suggesting that there was a division of Terminators ready to teleport into the battle, and the Chaos Lord could not be seen. If those elite troops were to arrive behind his advance the battle could be over. However, once he saw the Terminators teleport into position to shore up the faltering Iron Warrior line he pushed the attack.
Iron Warrior assault troops came under heavy fire as they moved to engage the right flank, and by the time they reached combat they were significantly outnumbered. A small force of drones moved to assist and dispatched them, allowing the Fire Warriors to continue their advance. The Terminators then came under heavy fire, all but annihilating them. Those that survived unleashed a brutal counter attack, killing half the Fire Warriors on the left flank with close range flamer blasts before engaging in combat. The same drone unit that defeated the previous assault moved in and eliminated them.
The remaining Chaos infantry made a valiant last stand, trying to hold back the Tau advance, but Elan Ros forces concentrated fire and destroyed unit after unit. In the final moments of the battle the Chaos Lord appeared, but an accurate railgun shot sliced him in half. When the dust settled the Chaos force had been killed to a man. By comparison the Tau had suffered minimal losses. A third of their transports had been destroyed, and another third damaged, along with a little over a quarter of the Fire Warriors. Their armoured and suit units were untouched, having suffered no losses in the battle. Elan Ro’s advance had taken the Tau to within striking distance of Montgate.
While Elan Ro showed the full flair of a Tau army advancing in sweeping battles of manouevre, Commander Fastblade was allocated a very different, and in some ways more difficult task. The veteran of the Vastrid Campaign was ordered to take Montgate once Chaos resistance south of the city had been wiped out. On 0103.009M42, Fastblade got his chance.
Elan Ro had opened the way for Fastblade's army, of similar size, to attack the city. The problems faced by Fastblade were all too obvious. Montgate was defended by deeply entrenched Iron Warriors who had no intention of abandoning their stronghold, and the city was flanked by wastelands and the coast to the east and south, and the high crags of the Mertock mountains to the north and west. The only option was a direct assault into the Chaos guns. Not a prospect relished by most Tau commanders.
As ever however Shadowstrike had chosen well in his selection of the conservative and patient Fastblade. The Tau commander moved his forces to the endge of the city by night, minimising casualties, then began the methodical process of destroying the Iron Warriors, and much of the city, with indirect firepower.
The Tau had overwhelming advantage in munitions and in just tow days much of the city had been levelled and its defenders forced to retreat to the Mountains. Depsite a last ditch deepstrike assault by Terminators and a Chaos Lord himself, the Tau took few casualties and took the city, or rather the rubble, in four days. Fastblade then pushed up into the mountains to prevent the city coming under direct fire from the remnant Chaos forces, then paused to rest, his mission complete.
Tau strike west
While Elan Ro and Fastblade forced the Iron Warriors into an irevocable defeat, Shadowstrike himself led an attack on the HQ and supply base for the northern imperial pocket. A specialist infiltration and special ops cadre was inserted by a low flying orca, bypassing the front lines and striking without warning at the Porphyrian command centre. It was lightly defended but the Porphyrians were unexpectedly well organised and re-enforcements arrived early in the operation.
Seeing that more forces would be needed to fully overwhelm the northern pocket and confident in his subordinates ability to carry the battle and capture the imperial base, Shadowstrike withdrew to call in further hunter cadres under the command of Elan Ro. After this action questions remained surrounding the conduct of the Tau commander in battle, as Imperial reports suggested his withdrawal was indeed swift and occurred after he came under heavy fire...
Closing the Gap
Shadowstrike’s drive into the Imperial beachhead in the north had fallen short of where he had hoped. There was still a thin strip of land through which the Imperial’s could communicate, a gap that needed to be closed. Shas’vre Ol Cha, a member of one of Elan Ro’s northern Cadres, volunteered to take a small strike force and close the gap.
Thus the Shas’vre, along with a mechanised unit of Fire Warriors and a small contingent of Stealthsuits raced towards the sea. They found their opposition on the coast in the wreckage of a Gue’la recreational facility, a platoon of Porphyrian Imperial Guard with a small mechanised contingent had been left to hold the line. Ol Cha’s force crept in close before engaging, the Stealthsuits engaging on the left, leading half the Porphyrian defenders on a merry dance as they cut them down with coordinated burst cannon fire.
On the right the Porphyrian’s put up much stiffer resistance, counter attacking with their mechanised elements, succeeding in immobilising the Devilfish, destroying its drones and driving back the Fire Warriors. However, Ol Cha then directed a renewed offensive, slaughtering the attacking unit and destroying their transport. All that remained was the lieutenant in command of this gap and his command section. Ol Cha offered a chance to surrender, but the lieutenant refused. They were killed to a man.
The mission was a complete success, with the enemy wiped out to a man, no Tau lives lost and only minimal equipment damage. Following units shored up Ol Cha’s gains. With the Porphyrian’s now cut in half and their supply base denied they had little choice but to surrender. The Chaos forces took this opportunity to seize a little more land, which the Tau allowed, feeling no need to expend the effort to deny it to them. There were no longer any Imperial forces in northern Merrin. Ol Cha received a promotion, taking Yr Doran’s position as Elan Ro’s second in command.
With the Merrin mines taken, and the Imperial beachead cut in two, there was no hope for the remaining Porphyrian regiments which had landed only weeks before. Commander Saladin himself was evacuated, wounded, to a hospital frigate offshore, while his troops desperately tried to re-embark on their transports. There were too few, and relentless bombing by the Tau air Cadre sank many of them, including Saladin's hospital ship. He was never found.
Leaderless and defeated, some 20,000 men, nearly two regiments of Porphyrians mutinied. Refusing to fight to the death they killed their commissars and then surrenderred to the Tau. The surrender was accepted and all those willing were pressed into the Tau's forces. The regiments were split up, being attached to Tau cadres and deprived of their heaviest equipment to fight as Gue'vesa auxilaries. The Porphyrians were now down to one third their original strength on Tarsis and had lost almost all their senior ranking officers. General Du Prez, dejected at such a sudden reversal of furtunes, pulled all the remaining Porphyrians out of front line operations.
The southern advance
It was actually the advance by Or'es Kair, the least experienced commander, which started the momentum of the Tau in the south. Shadowstrike had called for a limited encirclement against the mighty river Fuller, but as Kais moved her forces forward, she found the defences rather lighter than anticipated. Cautiously the Tau continued to advance, but critically, kept their flanks from being exposed. Then the Ultramarines sprung their trap.
The Ultramarines had intended to blunt this expected Tau attack by arranging for the Imperial Guard to fall back. this would encourage the Tau on then, when they were over stretched, the Adeptus Astartes would cut their supply lines and "pinch off" the Tau salient. However the plan began to fall apart immediately.
For a start not all the Imperial regiments understood the plan. On the right flank the Librians and Trebbians fell back, but the Tarsis PDF units, out of the loop, did not. They stayed put and assumed the Librians had retreated. Some 300,000 PDF troopers now found themselves in a pocket. If the Ultramarines plan went wrong, they would be captured.
It did go wrong. The Tau of Or'es Kais fought much harder than expected and displayed an excellent tactical appreciation for the super humans' tactics. In under two days the Ultramarines, taking heavy casualties once again, were forced to withdraw. This they did without consulting General Cochrane, which ensured the PDF in the pocket had no chance of withdrawing. Virtually all of them, and their equipment were captured by Kais' smaller force, and a huge area to the east of the Fuller river fell to the Tau with East Plainbrook less than one hundred miles away. General Du Prez hurridly signalled his subordinate, who stated that with the losses on the Fuller plain, he would be pushed to hold the city. Cochrane requested to pull out and form a defensive line in the South Shore Peninsular. His request was denied.
The Siege of East Plainbrook
Following the dramatic and astonishing advance by Or’es Kais, the Tau brought forward their timetable, moving their forces quickly and efficiently, deploying Elan Ro’s main force, which has been in the north, south to take advantage of the lightning advance, not allowing Cochrane’s battered forces to regroup.
Despite being in a position to attack Montgate that honour, as planned, was given to commander Fastblade. Instead Elan Ro, along with the majority of his force, moved south, to East Plainbrook. Or’es Kais, a new commander, had made a significant push south, putting the Tau in a position to take the city. Librians were entrenched in the city, and Elan Ro was more than willing to dislodge them.
General Cochrane, having lost almost a fifth of his forces thanks to the Ultramarines’ collapse some days earlier, hoped to slow up the Tau by forcing them to take East Plainbrook, at great cost. He knew the Tau disliked fighting in cities, and entrenched a large force in the city while he tried desperately to reorganise his Trebbian and Librian units to defend against this new Tau offensive. Imperial High Command could promise little in the way of support, as they were now hard pressed by the Orks of Snazteef in the north of Tuva, and were still recovering from defeating Chaos in the south. Now Elan Ro would take advantage of the Imperium’s compromised position.
His forces drove hard and fast into the city, with deep infiltrating Stealth units engaging the Librian heavy artillery and destroying it before a shot could be fired. Half the Hammerheads were tied up in a battle with entrenched Librian Leman Russes, becoming immobilised before destroying the Gue’la tanks.
After accomplishing their mission the Stealth units began to fall back, tacking heavy casualties. The Librian units on the left flank came under fire as Elan Ro tried to support the Stealth units, driving them off, but not before the Stealth units took over fifty percent casualties and moving at an exceptional pace rapidly disengaged.
A flanking unit of Storm Troopers caught a section of mechanised Fire Warriors completely off guard, annihilating their transports before assaulting the survivors of the crash. Other Fire Warrior units then pinned the Storm Troopers in place before they pulled back. Deeper in the heart of the city a vicious tank battle had begun. A division of Leman Russ tanks engaged mechanised Fire Warriors, blowing Devilfish out of the sky, before a unit of Hammerheads destroyed them with accurate railgun fire.
Some Hellhound flame tanks were looping round to engage the main Tau advance. Knowing the threat these vehicles possessed the immobilised Hammerheads opened fire the moment they entered line of sight, igniting the fuel tanks and destroying them in an enormous fireball.
Deep in the bowels of the city the real brutal fighting had begun. Fire Warrior and Crisis Suit units, covered by Hammerheads engaged the bulk of the Librian infantry. Mortar units took a heavy toll on the Fire Warrior units, before being silenced by Crisis Suits. A Storm Trooper mechanised counter attack faltered, and was promptly crushed. However, the reserve Librian infantry units caused massive casualties on the Fire Warriors and eliminated the forward Crisis Suits, including Yr Doran, Elan Ro’s second in command.
The Tau had managed to secure the edge of the city, despite a valiant effort by Storm Troopers to cut off the Tau’s access route in the closing hours of the first day, but the heart was still firmly in Imperial hands. Elan Ro pulled back his forward units as night fell on the city. Throughout the night there was some sporadic fighting, as Imperial Storm Troopers and Tau Suits tested the enemy lines, but no real gains were made.
As dawn broke it became apparent that the majority of the Imperial vehicles had been destroyed on the first day. The Tau vehicles had also suffered heavy losses, with most Devilfish destroyed, and those that remained were heavily damaged. However, there was still a functional division of Hammerheads, giving the Tau some serious mobile firepower. Thus began the task of sweeping the city street by street.
Under the cover of Hammerheads and Crisis Suits Fire Warrior teams would advance and enter buildings. Stealth teams would land on the rooftops and begin their cleanse from there, trapping the Imperial forces on the middle floors. Once the Imperials were pocketed the Hammerheads would slam railgun shots through the floors Imperials were on. It was a painstakingly slow process, and sporadic counter attacks from Storm Troopers that had to be met and seen off with Crisis Suits slowed the process further, but the Tau were advancing. It took a further four days to complete the sweep of the city, the Librians fighting bitterly for each and every street, but as the sun rose on the sixth day the city of East Plainbrook was under Tau control.
The attack on East Plainbrook, though successful, had stalled the Tau advance and proved once again that when forced into a battle where manoeuvre was difficult or constrained, the Tau found it hard going. Shadowstrike had allowed for this, but though he was pleased by the capture of the Imperial city, he had hoped for more. His forces had not yet reached the coast. Having planned for this, a force under Shadowstrike’s command was already sweeping south, bypassing the vicious fight in the city.
South Shore falls
In the south, by 2502.009M42, a mere five days into the operation, the situation was far better than the wildest of Tau hopes. Or'es Kais' forces were spent however, and could not press their advantage and Elan Ro was pinned down in heavy fighting on the streets of East Plainbrook. While the Imperium was struggling to pull together enough forces to hold the city, Shadowstrike saw an opportunity to deal a death blow to imperial efforts on Merrin.
Shadowstrike transferred his forces south by manta to the south of Elan Ro's position, which by this point had moved into the centre of East Plainbrook. There, bypassing the city's main defences, infiltration cadres slipped through gaps in the chaotic front lines to strike critical strategic points throughout the south shore region. These were defended by the Trebbian 9th but they mostly consisted of rear echelon units, poorly supported and equipped with few armoured vehicles. They suffered repeated heavy defeats and all resistance in the area collapsed by 0303.009M42, leaving a small pocket of Imperial resistance led by General Cochrane and his Librians on the eastern shore of the continent.
Outcome
Operation Poseidon eventually ended on 0603.009M42, not because of effective enemy resistance but because the Tau were exhausted. There were no more fresh cadres to throw into gaps in the enemy lines and supplies of ammunition and fuel were almost exhausted leaving the tau incapable of mounting further offensive opperations for the time being.
The Tau offensive ended though mopping up continues and the fronts were fronts secured. An equally ambitious operation then began to fly in supplies and ammunition from the orbiting fleet and rotate out exhausted cadres for fresh ones recuperating on fleet transports and rear echelon areas. Poseidon was pronounced an overwhelming success with gains beyond the Tau's wildest expectations and a delegation from the ethereal caste sent its congratulates personally to all the Tau commanders on Tarsis Major. Almost immediately, HQ staff and intelligence analysts begin planning operations to take the war onto Tuva...
The effect on the Imperium was no less dramatic. Stunned by a 60% gain in Tau territory in just over a week, Du Prez was chastised by senior command. Du Prez blamed the defeat on the fact that the Tau were still able to ship in fresh supplies and troops, so the sector commander, Thaddeus Roover, approached the Imperial Navy. Within 48 hours Jellicoe's grand fleet was sailign for Tarsis, and a new battlegroup was arriving in system from hubward. The Imperial fleet was ordered to annihilate the Tau once and for all, blockading and trapping the aliens on the planet.
General Cochrane meanwhile was stuck, defending a small pocket of ground on the Merrin coast with just over five divisions. His supply lines untenable, he was still expected to fight to the last. The force, mainly Trebbian and Librian, was ordered to reject the surrender terms proposed by Shadowstrike. Cochrane then attempted to negotiate a withdrawal from Merrin.
Elan Ro was keen to accept this, but Shadowstrike overruled him. He could not allow five divisions and their equipment to be redeployed as they would need to face them again in subsequent fights. The negotiations were ended and the Tau prepared the final massacre.
The Porphyrians were badly mauled, with 10 of their 15 original regiments on Tarsis destroyed or captured by the Tau. Almost two regiments switched sides, swearing loyalty to the Tau. Fearing retribution the new commander, Colonel Abbas, immediately reaffirmed Porphyria's and Porphyrian's loyalty to the Emperor. Even so the shattered units of the Porphyrian Guard were gratefully removed from front line duties for the time being.
While the Iron Warriors dug in on Tarsis, their commander was able to smuggle out a small forces and link up with loyal Chaos forces in the Tarsis system. From here a rag tag band of forces travelling in small Iconoclast and Infidel raisers travelled to the Rim Worlds looking for support for their cause, and anything which would prevent their wholesale destruction on the planet. The Iron Warriors were not about to leave Tarsis Major, they would do anything to fortify their position and wait, for centuries if necessary, for reinforcement. 
