According to Davros, speaking to the Tenth Doctor in "The Stolen Earth", "Emergency Temporal Shift took him back into the Time War itself... it cost him his mind".
The Doctor told Davros, "In the very first year of the Time War, at the Gates of Elysium... I saw your command ship fly into the jaws of the Nightmare Child. I tried to save you." The Doctor failed to save Davros from the Nightmare Child but Caan succeeded.
Davros thought that Caan was helping him & the Daleks to destroy all life -- other than the Daleks -- in all universes, using the "Reality Bomb". In "Jpouney's End" (the 2nd part of the 2-part story), it was revealed that Caan had been working against Davros & the Daleks. He had, in fact, arranged for Donna Noble to become "the Doctor-Donna" & to defeat them, as the dialogue eventually explained:
- DOCTOR (to "the Doctor-Donna"): You're so unique the timelines were converging on you. Human being with a Time Lord brain.
- DAVROS: But you promised me, Dalek Caan. Why did you not foresee this?
- DOCTOR: Oh, I think he did. Something's been manipulating the timelines for ages, getting Donna Noble to the right place at the right time.
- CAAN: This would always have happened. I only helped, Doctor.
- DAVROS: You betrayed the Daleks.
- CAAN: I saw the Daleks. What we have done, throughout time and space, I saw the truth of us, Creator, and I decreed, no more!
It's worth noting that Caan's decree "No more!" is exactly the same as that of the War Doctor (in "The Day of the Doctor") when he decided to end the Time War, even at the cost of destroying the Time Lords, as well as the Daleks.