Beginning in 2525, Harvest was the first world to fall to the Covenant's genocidal wrath. By 2535, with the destruction of Jericho VII, most Outer Colonies had been lost. By 2552, though some Outer Colonies still remained in UNSC hands, most were small and rather insignificant worlds. The significant colonies still under UNSC control lived in constant fear of Covenant invasion. When the Human-Covenant War had ended on March 3, 2553, tensions in the remaining worlds had presumably significantly died down.
[edit] Known Systems

Eridanus system

* Eridanus II - Planet glassed by the Covenant in 2530
* Eridanus Secundus - Asteroid with a rebel base, presumably destroyed by the Covenant in 2552

Lambda Serpentis system

* Jericho VII - Planet glassed by Covenant in 2535

Chi Ceti system

* Chi Ceti IV - Planet abandoned by the UNSC in 2525

Epsilon Indi system

* Harvest - Planet glassed by the Covenant in 2525

111 Tauri system

* Victoria - Planet with unknown status

Alpha Aurigae system

* Origami Asteroid Field - Asteroids with unknown status, attacked by the Covenant in 2526

Procyon system

* Arcadia - Planet attacked unsuccessfully by the Covenant in 2531, but attacked again and destroyed in 2549

23 Librae system

* Madrigal - Planet glassed by the Covenant in 2528
* The Rubble - Abandoned and destroyed except for Habitat Exodus

Lambda Aurigae system

* Roost - Planet with unknown status

XI Boötis A System - System still held by UNSC

Groombridge 1830 system - System attacked in 2530

* Groombridge-1830

Beta Centauri - System with unknown status

Biko - Planet glassed by the Covenant on an unknown date before 2534

Coral - Planet glassed by the Covenant in September 2552

Dwarka - Planet presumed glassed by the Covenant, as all contact was lost

Emerald Cove - Planet with unknown status, abandoned by the UNSC in 2542

Hat Yai - Planet glassed by the Covenant in either 2528 or 2529.

Paradise Falls - Planet with unknown status