Editing The Tartaros Campaign

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{{Infobox 40k Campaign
|title=the Tartaros Campaign
|image=
|attacker= '''Aggressor:''' <br />Imperium of Man
|defender= '''Defender:'''<br />Messonian Empire
|commander1=Aldous Cadigan<br /> 
Oskar Reynhardt<br/>
Karolus Vayne
|commander2='''"Emperor" Bassal-Narassur XVI'''<br />
General Stasanor Molon<br>

|date=813.M30
|scale=across multiple sectors
|theatre=Messonia, Illiria, Micaea, Syrene, Uphrates and Ionyan sectors
|strength1=Full XIII Legion forces<br />
Over 900 million Imperial Army and Auxilia <br />
several full strength naval battlefleets (est. ~6,000 craft)
|strength2= ~11 billion soldiers <br> 
~15 billion volunteer forces from the "city states" <br />
Vast naval fleets
|casualties1= Unrecorded but substantial. Estimated at ~11 billion combatants <br />
Over 12,000 void-craft recorded as destroyed or captured
|casualties2= 5,782 Astartes<br />
386 million Imperial Army
|status=Decisive Imperial Victory
|outcome= 
}}

In 813.M30, the '''Justicars''', under the command of '''Aldous Cadigan''' were dispatched to conquer the '''Messonian Empire''', in the Tartaros cluster. By the standards set later in the Great Crusade, the Messonian Empire was only a small challenge, but for the fledgling Imperium it was a formidable rival.

Logistical accounts reveal that the campaign was expected to be long and bloody; the logisticians estimated the conflict's time frame to completion would range between 4 and 8 months. This investment of time was deemed economical, as the Messonian Empire had a flourishing and highly valuable industry, and crucially controlled an important warp nexus that could speed the Imperial push into the galactic North East and greatly strengthen lines of supply for future campaigns in the Ultima Segmentum. 
The Justicars were allocated the objective of taking it.

The Messonian Empire had an incredibly large population, the majority of its planets being what the Imperium would call 'hiveworlds', with the capital, Tartaros rivaling even the most impressive of Imperial hiveworlds. But where the Imperium would support these hiveworlds with sister-worlds dedicated to agriculture, the Messonians supported their empire via planetary subsistence farming; vat-grown meats and an high-yield fungi crop grown in titanic laboratories deep under the ground kept each planet fed.

With no way to starve the Empire in a siege, and facing a numerically superior foe, the 13th devised a stratagem to suit the circumstances. After a brutal and rapid strike across the Empire's fronts, the legion would committed its entire fleet to ensure orbital supremacy over the Messonian Capital planet, Tartaros. They then landed their troops, focusing their assault initially on the capital, Tartaros, then expanding to assault 8 more cities. Once the cities were taken the legion fortified every street and building and held their ground, waiting.

The Enemy was well prepared for the assaults, and put up a fierce fight, but with the orbital barrage supporting the attackers, they were forced to concede many cities. Regrouping their forces with consummate speed and skill, the Messonian Armed Forces encircled and besieged the city with large forces, that moved into position via underground tunnels. The Messonian forces deployed their troops so close to the Imperial frontlines that the ships in orbit could not fire on them without risking hitting their own forces too. 
The Messonians however, kept back considerable reserves, wary of a trap. The fighting in the cities was brutal and unforgiving, with each hab-block and warehouse becoming a citadel; areas of the city were lost and retaken several times a day. But slowly and surely, the Imperials were pushed back. 
2 of the 9 besieged cities were abandoned as the legions forces retreated back into the safety of their orbital blockade, and one city was lost entirely; now convinced of their superiority, the Messonians began to commit more men to the assaults on the remaining cities.

It was  in the second month of the campaign that, having drawn the majority of the enemy forces into battle, and lulled them into a false sense of security and superiority, the Justicars finally revealed their true strategy.
New forces deployed from orbit, using drop pods and aircraft to deploy troops behind the enemy lines, whereby the tunnels were collapsed. Fierce battles raged as the Messonians realising they were now trapped sought to break out to have a chance at survival, but were met time and time again with brutal ambushes and deadly traps.
Those resigned to their fate in the tunnels were given an ultimatum: Surrender or die by suffocation in the tunnels. In droves the Messonians surrendered. But many held their ground also, resolved to defend their empire until death. These valiant defenders fortified the tunnels expecting an assault, but none came.
The 13th collapsed the tunnels on their frontline, leaving behind only promethium tanks with explosives rigged to them to make powerful incendiary bombs. These bombs detonated, doing little damage to the enemy forces, but the fireballs the created consumed all the oxygen in the tunnels in a sudden violent burst, and in short order the defenders died, either attempting to flee to the surface to be cut down by power-armoured marines, or by suffocating in the tunnels.

After seeing how outwitted they had been, how ruthlessly their forces had been dispatched, and how effective the Astartes truly were when on the offensive, the Messonians capitulated. While the military may have surrendered, the fight was not over in the minds of many Messonians; Generals, soldiers and civilians alike formed anti-Imperial insurgent forces to continue the fight, believing such unconventional tactics would allow them to outlast and defeat the direct nature of the Astartes.
But this course of action was a hopeless endeavour against the Justicars, for they were masters of counter-insurgency, and with their full expeditionary manpower the Justicars prosecuted a lightning campaign against the insurgents. Within a week, the entire sector had been purged of all anti-Imperial sentiment by a combination of air and orbital strikes, raids, and starving the insurgents of their popular support, isolating them as the populace was won over by Balotin's hearts and minds efforts. A program of reconstruction of the planets infrastructure, military, and economy began seamlessly on the completion of this counter-insurgency operation, and the Messonian people were thoroughly won over by the Imperium, and would remain staunchly loyal to the Emperor for many millennia.