During the Great Crusade, the world of Tesstra was part of an alliance of totalitarian Human colonies united against Imperial authority. Known as the Conservation, this alliance that oversaw 11 billion people had survived the Age of Strife through strict eugenic control and a caste system. When Imperial emissaries were fired upon trying to negotiate with the Conservation, Warmaster Horus diverted two Expeditionary Fleets to bring the Conservation to compliance. One was to be commanded by Roboute Guilliman, another by Alpharius.
The Ultramarines under Guilliman began to immediately conduct a campaign to conquer the outer worlds of the Conservation, while Alpharius and his Alpha Legion chose not to immediately attack. They instead launched probing operations, reconnaissance missions, and raids. Infuriated by what he considered the Alpha Legion's lack of progress, Guilliman berated Alpharius at council and demanded Alpha Legion forces be placed under his command. Alpharius simply ignored his fellow Primarch. Shortly thereafter, Alpha Legion bypassed the Ultramarines lines and attacked the inner Conservation world of Tesstra. But again this attack was seemingly slow and sporadic, and the Conservation troops led by genetically modified Ogryns were able to dig in and hold onto Tesstra's Prime city of 60 million. For more than a week, Alpharius delayed a full-scale invasion as more and more Conservation forces were deployed around Tesstra Prime's capital in anticipation of the inevitable attack.
However when the Alpha Legion's assault finally came, it was from a hundred different vectors and saw Alpha Legion infiltrators sabotage points inside the city walls. The city lost power and its defenders became paralyzed when bridges and communication stations were detonated by hidden explosives. With forces both inside and outside the city and the defenders completely paralyzed, the Alpha Legion slaughtered all before them with a spearhead of Fellblade and Typhon super-heavy tanks. The Conservation defenders were ruthlessly extermination and civilians clogged the roads attempting to flee, only to be cut down by Thunderhawks and Fire Raptors. After two days of anarchy and panic, a weak counter-attack was organized by the Conservation led by their Ogryn soldiers, but Alpha Legion informers exposed their plans in advance and the genetically modified Abhumans had their officers slain by snipers. The leaderless Ogryns were subsequently torn to pieces by squadrons of Sicaran tanks. The battle won, the Alpha Legion withdrew, leaving the city a ruin and 90% of its population dead. The rest of Tesstra surrendered shortly after the example made.
Later, Guilliman asked Alpharius why he did not simply seize the city rather than allow the Conservation to reinforce it for days. Alpharius simply stated that it Would have been too easy.