Chaktra

The Chaktra were a war-like race who were engaged in multiple conflicts throughout the galaxy.

One day, they came to the Telphin homeworld. Because of a garbled report, they took them to be allies of their enemies. They bombarded the planet, killing all the Telphin in a single day.

The Seventh Doctor later recounted this story to Simon Galway to prove that war was unjustifiable. (COMIC: Memorial)