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 did the Daleks and the Timelords actually fight a time war? Did they send their forces deep into the past of your enemy? Change history so that your enemy is weaker or perhaps doesn't exits?
Certainly, the TV story "Genesis of the Daleks" is taken to be the first step in the time war by the Time Lords. But what about the Daleks?
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.