The Etra Prime Incident was the Daleks' first attempted invasion of Gallifrey. (PROSE: The Apocalypse Element) It indicated that the time had come for the Time Lords to abandon their non-interference policy, foreshadowing the War (PROSE: The Ancestor Cell) and beginning the escalation of hostilities between the Daleks and the Time Lords. (PROSE: The Stranger)
The incident began when the Daleks removed the planet Etra Prime from space-time, kidnapping a Monan and Time Lord delegation including Lady President Romana. The Daleks removed Etra Prime so that they could harvest the Apocalypse Element from Etra Prime itself.
Twenty years later the Daleks set Etra Prime on a collision course with the planet Archetryx, which was hosting a temporal treaty attended by twenty member races of the Temporal Powers. Before Etra Prime collided with Archetryx, President Romana escaped from Etra Prime and warned the Time Lords of the threat, allowing them to retreat to Gallifrey.
The Daleks followed the Time Lords back to Gallifrey and lowered the transduction barriers, allowing them to invade. The squad of Daleks who first followed the Time Lords landed on Gallifrey with a larger force set to follow later. The Dalek squad made their way to the Panopticon to seize the Eye of Harmony. The Daleks used the Sixth Doctor's companion Evelyn Smythe to open the Eye of Harmony, but were stopped by the Doctor and Romana, who forced the remainder of the Dalek squad into the Eye of Harmony with the help of the Chancellery Guard. The Time Lords restored the transduction barriers before any more Daleks arrived. (AUDIO: The Apocalypse Element)