Harmony

Harmony was a human colony that was glassed by the Covenant Navy in 2552. It is unrelated to New Harmony, another UNSC colony.

Human-Covenant War
In February 2552, insurrectionists on Harmony disabled the planet's primary communications relay and stole two freighters from a drydock.

Several months later, Colonel Herzog suspected the Office of Naval Intelligence had broken the code used by the Covenant and learned of the upcoming attack but let Harmony fall, along with Troy, to ensure the Covenant wouldn't realize. It was implied he was right, saving countless lives.