Etra Prime incident

The Etra Prime Incident was an event that began the escalation of hostilities between the Daleks and the Time Lords in the events leading up to the Last Great Time War. (DWAN: 2006 Doctor Who Annual)

History
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 sent 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 managed to escape from Etra Prime and warn 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 access to Gallifrey 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, who forced the remainder of the Dalek squad into the Eye of Harmony with the help of the Chancellery Guard. The Time Lords managed to raise the transduction barriers before any more Daleks arrived. (BFA: The Apocalypse Element)