Troy

Troy is a UNSC colony world located in the Hellespont system.

Human-Covenant War
The  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.

Trivia

 * The last ILB Axon chapter contains the start of the Covenant invasion of Earth, a set date, and each chapter is set a week apart. In the first chapter, axon receipt, Herzog refers to the evacuation as "three months" ago.