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Galaxy 5 was a terrorist organisation which opposed the Galactic Federation.

In 3935, the Federation and Galaxy 5 were at war. Galaxy 5 sent agents Azaxyr and Eckersley to take over the trisilicate mining on Peladon, which was vital for the Federation's war effort. After the failure of this mission Galaxy 5 quickly agreed to a peace treaty. (TV: The Monster of Peladon, PROSE: Legacy)

Torin Chalfont had reported from the front line on the first Galaxy 5 war. In 3999, Galaxy 5 was still a terrorist threat. (PROSE: Placebo Effect)

On-screen, Galaxy 5 was not identified as being an organisation rather than an actual galaxy; it was retconned as such in the novel.
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