Tachi was one of the Zeon soldiers who took part in an attack led by Crowley Hamon on White Base in order to avenge Ramba Ral. While Hamon commanded the remaining Magella Tops, Tachi piloted the single Zaku left in Ral's forces. They attempted to ram an explosive-filled Gallop hovercraft into White Base, but Amuro went out in the RX-78-2 Gundam to stop them.
As Tachi was attempting to get onboard White Base with his Zaku, he was called back by Hamon to assist her other forces in repelling the Gundam from the Gallop. Tachi then engages the Gundam, but Amuro manages to knock him into the blast of one of the Magella Tops, resulting in Tachi's death and his Zaku's destruction.
Principality of Zeon