On 0511.008M42 General Cunningham launched his much expected major assault against Chaos forces on the southern tip of Tuva. With the recent arrival of strong Daemonhunters and Dark Angels units, Cunningham now felt he had the tools he needed to attack on a broad front, utilising the Dark Angels and other elite forces to crack the difficult nuts in the Chaos defence, while his own Guard forces would engage the weaker and less well equipped cultist legions.
The Catechism was off balance fighting Orks in the east, and the Night Lords had just recovered from the Tau attack on their base on Typhon, and significant resources had gone into their counter invasion of Rhesisus. Cunningham, Lord General Du Prez and the senior Commander of the Dark Angels all agreed that now was the ideal time to strike.
The Chaos forces on Tuva actually struck first, with Iron Warriors making progress on the north eastern end of the Barrier Mountains, before the major advance by Cunningham's army began. Then, as Guard units swept down to the sea, the Dark Angels encircled an entire Chaos army in Maybourne and wholesale slaughter began. The Daemonhunters too went straight for the city, intent on ridding it of the Chaos taint, and the two elite Imperial forces, their enemy surrounded them, hunted them like prey. By 1012.008M42 the defensive axis of Chaos on southern Tuva had been broken and the Imperial Guard had forced cultist forces back to the entrance to Sapphire Harbour.
Meanwhile in Maybourne the last elements of Chaos forces, both traitor marine and traitor guard units, had been annihilated. Then the ugly spectre of "cleansing" by the Inquisition began. Maybourne had been a city with a population of 10 million before the invasion, and the 4 million who had managed to survive the Chaos occupation soon wish they hadn't, as a further 2 million were executed at the merest suspicion of being tainted. Even so the Imperium hailed the offensive as a great victory and the bells of Tarsis Prime were rang for the first time since the invasion.