Rhodian-Quill Civil War

On the planet Rhodia, a civil war broke out between its two inhabiting races, the Rhodians and the Quill.

Factors that led to the conflict
According to the Rhodian, Charlie, the Quill mismanaged their economy, and grew resentful of the Rhodians' success. However, Andra'ath claimed that the Rhodians ate up all of the planet's resources, leaving them with little to work with and no way exist without being dependent on the Rhodians.

Charlie believed that the Rhodians did their best to help with the situation, but Andra'ath argued that in doing so, the Rhodians had put the Quill in a situation that they could never be able to properly recover from. (TV: For Tonight We Might Die)

Regardless of either species' perception, the two were eventually at war with each other.

The Civil War
To be added

During the conflict, the Quill were approached by beings from a nameless, faceless organisation (later responsible for the Skandis War) with an offer to help them in their fight. The Quill were unable to accept this offer as the organisation asked for more money than would be possible for them to provide. (PROSE: What She Does Next Will Astound You)

Aftermath
After the war had ended, the main Quill resistance were enslaved via the use of Arns that telepathically bound each Quill to a particular Rhodian.

Some time later, whilst both sides were tired and lacking in resources as a result of the war, the Shadow Kin invaded and wiped out the population of the whole planet, apart from only two survivors - Charlie, the Rhodian prince, and And'rath, the Quill's leading freedom fighter - who were rescued by the Twelfth Doctor. They were delivered to Earth and told to use Coal Hill Academy, originally as their cover for hiding from the Shadow Kin but later as a point of defense from space-time tears & the threats that could come through them. (TV: For Tonight We Might Die)

At some point during their time at Coal Hill Academy, Andra'ath - now referring to herself as Miss Quill, in honour of her lost people - was able to show Charlie how the Rhodians had put the Quill into a state of poverty. He was shocked to see that he had been at least partly unaware of the situation on the Quill side. (AUDIO: Mock)

After about four months on Earth, Andra'ath had her arn removed and was free from slavery. After this, she vowed that the war would begin again, referring to both her anger about the Rhodian souls having been preserved in the Cabinet of Souls, and her desire to kill Charlie. (TV: The Lost)