It is frequently said that the time war exists outside of time in a time lock; that means the time war is happening at all parts of time. The only Time Lord who can bring them out of the time lock is Omega, the creator of time travel. Also it's called a time war because they don't actually die, they just keep on killing each other forever. We see this in the episode "End of Time" when the doctor talks about how they got creative at killing. We also see the "fury of the time lord" in The Family of Blood. Where he forces the family to suffer until the end of time.
The actual time war was started because the Time Lords saw that the Daleks would destroy everything in the Universe, and so they sent the fourth doctor (Tom Baker) to destroy them before they begin. We see this in the classic episode "Genesis of the Daleks". The Daleks then start war with the Time Lords for doing this and as a result the war has no end so the Doctor must put the war in a Time Lock to exist outside of time itself and leave home.