When Doctor Who was rebooted in 2005, one of the main themes since then was the "Last Great Time War" that pitted the Daleks against the Time Lords.
But how does one actually fight a time war? Do you send your forces deep into the past of your enemy? Change history so that your enemy is weaker or perhaps doesn't exits? Wouldn't this cause such paradoxes that the Reapers (introduced in Episode 8 of Series 1, Father's Day) to try and restore time?
Certainly, according to this link, the "Order of the Black Sun", sent an agent back in time to just around the time the Time Lords were starting their time travel experiments, only for the event to help them establish time travel.