Attack on Helatrine

The attack on Helatrine was waged by the Daleks in the midst of a war with the humans. As the war was taking its toll, the Daleks sought to acquire Helatrine's bio-psychic weaponry and so conquered the planet. (PROSE: Infection of the Daleks)

Behind the scenes
The Daleks use of Ogron slaves in a conflict with humanity which is "taking its toll" would implicitly place this event during the Second Dalek War.