Sontaran

The Sontarans are humanoids from a high-gravity world, with a squat build and distinctive dome-shaped heads. They were originally a conventional two-gendered species, but early in their history as a space-faring people (approximately 1400 BC Earth time) they shifted over to a clone-based method of reproduction. This was a result of their government's takeover by their highest military official, General Sontaris, who used newly developed bioengineering techniques to clone himself millions of times over. Sontaris annihilated the non-clone population and renamed the species after himself. All Sontarans since then are derived from the genetic stock of General Sontaris, though some modifications have been introduced over time.

General Sontaris' militaristic outlook forever altered the Sontarans' civilization. They became an expansionistic and warlike species, expanding slowly and crushing all other races they encountered in their path. Then, around 200 BC Earth time, the Sontarans encountered the Rutans[?]. The Rutans were as powerful and as aggressive as the Sontarans, and war broke out immediately. The war between the Sontarans and the Rutans has continued for at least five thousand years, with both sides remaining fairly evenly matched and neither side interested in negotiating for peace.

The Earth remained largely uninvolved with the Sontaran-Rutan conflict, with only occasional scouts and crash-landings by either side. At one point the Sontarans successfully invaded Gallifrey itself, but were driven out again after less than a day.