By 2005.010M42 General Cunningham knew that an Imperial fleet with reinforcements was just weeks away from reaching Tarsis Major. Confident that he would soon be relieved and bolstered by the arrival of no fewer than two companies of Space Wolves, Cunningham drew up plans for a new offensive on the continents of Merrin and Tuva. Meabwhile the Tau too had quietly received commander Moonshine and several new hunter cadres, managing to conceal this arrival through adept use of their stealth technology.
For once both sides managed to surprise their enemy. In the north east on Merrin the Cerberex Guard launched a ferocious attack on Strongspear's forces in the Mertock mountains. The initial attacks by the highly mechanised Cerberex formation destroyed much of Strongspear's tanks and broadside battlesuits in the opening engagements from 26-2805.010M42 and the tau commander was hard pressed to stop the Imperial regiments. Initially he tried to send in reserves, and though these fought bravely they met a similar bloody fate. Fire warriors fell prey to Demolisher siege tanks and Hellhound flame tanks, while those mounted in Devilfish transport faired little better as low flying Vendetta gunships proved almost impossible to counter in the steep sided valleys of northern Merrin.
Unable to bring his air superiority to bear in such terrain Strongspear was forced back, abandonning much of the north western arm of the Mertock mountains and retaking the fort once held by the Iron Warriors earlier in the war. On 3105.010M42 the Cerberex advance finally ran out of momentum, but the Impperium now had a secure bridgehead in the north west and most of the important towns were back in Imperial control.
On Tuva the Space Marines led the charge, breaking a hole in the Tau lines in the Downside Desert, taking the main water pumping facilities and the sparse settlements of the desert following rapid insertion operations. With the Astartes troops taking what were effectively the keystones of the Tau defence, the desert became an untenable position for the aliens. They were forced back and soon took up more defensible positions on the Tuva plain outside the capital Tarsis Major.
While the Tau were being forced back in the north, commander Moonshine was landing his troops in the lush forests of Sartano, much to the surprise of General Culden who commanded the small garrison on the continent. No one had expected an assault on Sartano and the Imperial defences were unprepared, a fact which immediately lost Culden his job. After replacing his commander in the field Cunningham ordered the Prozan Cavalry to throw the Tau back into the Maybourne straits. The Prozan regiment was ideally suited to such an offensive and they deployed effectively using their valkyrie assault carriers to approach Moonshine from the heavily forested Sarton heights. Initially the assault went well, the Imperial mobility proving exceptionally useful against the Tau. However Moonshine's force, which contained a high proportion of Kroot mercenaries, was ideally suited to the terrain, and the Prozan's accuracy of fire was somewhat lacking. After five days of sporadic and highly mobile fighting the Tau clearly had the upper hand and the Prozans retreated to positions around the vital Industrial heartland of the continent.