From Halopedia, the Halo wiki
The Inner Colonies were the first worlds terraformed and colonized by the Unified Earth Government, relieving the overpopulation pressure on Earth that resulted in the Interplanetary War in the 2160s. Collectively, Earth and the Inner Colonies are known as the core worlds.
For a list of notable Inner Colonies, see here.
Settling and formation
The Inner Colonies consisted of the 210 worlds which were colonized during the UEG's first wave of colonial expansion between 2362 and 2390, starting with the launch of the Odyssey and its colonization of the Epsilon Eridani system. Although volunteers for the colonization programs were plentiful due to Earth's deteriorated conditions, interstellar travel was still highly expensive and, as a result, the settlers of the Inner Colonies were subjected to rigorous physical and mental screening. This also led to the Inner Colonies being viewed as humanity's elite. By 2390, 210 human-occupied worlds were in various stages of terraforming and colonisation, including planets, moons, and asteroids. As the UNSC began settling the Outer Colonies, the Inner Colonies, with their established social and economic infrastructures, became the political and economic basis of the UNSC, supplied with raw materials from the newer settlements. By 2490, Reach, one of the first extrasolar colonies, had become the UNSC's military center.
It is known that there was a war some time before the 26th century called the Inner Colony Wars. Little is known about this conflict, except the fact that unrestricted news reports during the war caused "massive civil unrest" and threatened to "tear down Earth's culture".
During the Insurrection, numerous rebel movements sprang up on Inner Colonies such as Reach, initiating a wave of terrorism that saw many colonies as the scene of numerous bombings and massacres. The UNSC placed the elimination of these movements as a top priority in restoring order to the UEG's most critically important worlds, leading to the Spartan-II program.
During the Human-Covenant War, numerous ships were pulled from their positions in the Outer Colonies to defend the Inner Colonies. Old ships, such as the Halcyon-class light cruiser UNSC Pillar of Autumn, were refitted and brought back into service to supplement the remaining defense forces. A period of the war known as the Inner Colony Sieges began on June 1, 2536, lasting all the way until the climactic battles in the latter half of 2552.During this period, the war fell into a cyclic nature of human forces winning isolated battles on the ground, but taking huge losses in space. One by one, the Covenant began to exterminate the Inner Colonies. At the beginning of the Fall of Reach, a dozen Inner Colonies were bypassed by the Covenant fleet attacking Reach, and a number of Inner Colonies survived the war.
Government and politics