From Halopedia, the Halo wiki
The UNSC Apocalypso intercepted a Covenant transmission and was able to warn Troy of an incoming attack. Not wanting the Covenant to know they had broken their code, they did not defend the planet and managed to quietly evacuate some of the population before the planet was bombarded from orbit by the Covenant in May of 2552.[note 1]
The colony was named after the ancient Greek city of Troy.
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