Here is the intro I use for my Third Imperium, it is cut'n paste of various classic Traveller sources with some editing:
The Spinward Marches – the proto-Third Imperium introduction
The lmperium is a strong interstellar government possessed of great industrial and technological might, but unable, due to the sheer distances and travel times involved, to exert total control at all levels everywhere within its star-spanning realm. It encompasses 281 subsectors and approximately 11,000 worlds. Approximately 1100 years old, it is the third human empire to control this area, the oldest, and the strongest. Nevertheless, it is under strong pressure from its neighbouring interstellar governments, and does not have the strength nor the power which it once had.
On the frontiers the lmperium allows a large degree of autonomy to its subject worlds calling only for some respect for its overall policies, and for a united front against outside pressures. Extensive home rule provisions allow planetary populations to choose their own forms of government, raise and maintain armed forces for local security, pass and enforce laws governing local conduct, and regulate (within limits) commerce. To monitor the space lanes, the lmperium maintains a Navy. Because these forces can never be everywhere at once, local provinces (subsectors) also maintain navies, as do individual worlds.
Defence of the frontier is mostly provided by local indigenous forces, stiffened by scattered lmperial naval bases manned by small but extremely sophisticated forces.
Conflicting local interests often settle their differences by force of arms, with lmperial forces looking quietly the other way, unable to effectively intervene as a police force in any but the most wide-spread of conflicts without jeopardizing their primary mission of the defence of the realm. Only when local conflicts threaten either the security or the economy of the area do lmperial forces take an active hand, and then it is with speed and overwhelming force.
At the spinward edge, 120 parsecs from the original centre of the Imperium, the Marches represent one of the furthest extents of exploration and domination by Imperial forces. Lying adjacent to territory of the Zhodani Consulate and the Vargr Extents, this region is a site which has seen much conflict and intrigue.
The Regina Subsector
The hub of new development in the Spinward Marches is the Regina subsector. Located at the very edge of the Imperium, it serves as a contact point with the Vargr to coreward and the Zhodani to spinward; the result is considerable trade activity through the starports of the region. Much of the upper strata of Zhodani society are extensively trained in, and commonly practice the Psionic Heresy
The lmperium has been suppressing political dissent in order to keep peace in the Regina subsector.
A reward has been offered by the subsector government for the location of a senator who has been missing since 1102.
A recent uprising at Feri (0405) has cut the Imperial communication jump route from Regina (0310) to Efate (0105).
The government of Roup (0407) has made a subsector-wide call for surplus starships to supplement its local forces. There has been no opposition from the subsector government.
The Forboldn Project is the primary colonization project within the Regina subsector. Originally conceived in 987 to utilize the resources of Forboldn (0208), the project began its execution phase in 1089, shortly after the Fourth Frontier War. Large numbers of colonists were recruited and shipped in cold sleep from the Imperial core, with arrival times set from 1110 to 1120. Simultaneously, preparations on Forboldn began, with detailed planetary surveys to pinpoint resources and initial building projects to prepare industry and quarters for the arrival of colonists.
Interdicted worlds are interdicted because the lmperium is trying to conceal its mistakes in social and political planning.
Elsewhere in the Spinward Marches
Exile Camp. A location committed to the containment of individuals guilty (or presumed guilty) of political crimes of discontent. Governmental controls are usually restricted to the retention of inmates, and a general oppression of the population.
lmperial Prison. A penitentiary or rehabilitation centre for those guilty of lmperial crimes.
lmperial Reservation. A location, usually very large, under lmperial jurisdiction and restricted to use only by members of the lmperial family, or those authorized by a member of the lmperial family.
lmperial Way Station. A base established for the repair, maintenance, and overhaul of lmperial equipment. It may include provisions for Army troop barracks, naval and scout ship overhauls, and intelligence operations
The lmperium has long maintained (since 556) a research station in the Retinae system for the purposes of communications research. Accessed from Frenzie/Vilis via Thanber, the station has frequently figured in Zhodani diplomatic protests. Its long-standing presence at Retinae and the steadfastness of the Emperor has served to maintain its continued operation.
Asmodeus is recovering from a nuclear war which ended in 1005
World 728-907, recently surveyed, is a large inhabitable world with no evidence of higher animal life although extensive forestation and insect presence have been noted. The Ministry of Colonisation has designated the world for seeding within the next century, with a view to colonisation upon availability of personnel and funds.
Collace is the site of one of several lmperial scout bases in the district. Application has been made for membership in the Imperium, which is pending.
Tureded, a small agricultural world, has recently become of increasing importance as a trade and shipping centre because it lies at a junction for jump-I travel from rimward to the Regina, Jewell, and Rhylanor subsectors. It is expected that Tureded will be upgraded to a class B starport within the next decade. The Scout Service is currently negotiating the establishment of a Scout base, with the apparent intention of an xboat link from Rhylanor to Dinomn and Regina
The Ling Standard Products shipyards at Lunion and Strouden are the major shipbuilding points within the entire Spinward Marches. The excellent workmanship, combined with level D technology, makes LSP products highly sought after.
The asteroid belt at Zaibon was once the largest deposit of copper on record, but the lode has dwindled to virtually nothing, and the facilities are deteriorating.
Wardn is a small world notable primarily for its intricate patterns carved in its desert plains. One hypothesis holds the runes are marks left by anaerobic life, while another claims that they are artifact results of an ancient culture.
The government on Quiru is a military junta which is the result of a mercenary operation. Imperial force has not yet been brought to bear.
Egypt has been selected for a Ministry of Colonization training base.
Mithras is the site of an Imperial exile prison; convicted individuals are deported to the world where they begin life anew. Although environmental conditions are harsh, the opportunities on Mithras have made it a showcase of rehabilitation.
The Imperial Research Station at Duale has reportedly suffered extreme damage from an explosion of undetermined origin in 1102. A high degree of military security has been present in the system since that date. The nature of the research being undertaken is not known.
Nexine is an underpopulated water world currently being used by the Ministry of Conservation for reseeding efforts using biologically altered humans.
The desert world of Thisbe has undertaken a long-term project to divert large numbers of frozen water and gas asteroids from the Thisben belt to the planetary surface; the intention is an improved atmosphere and hydrographic percentage