The Two Dark Ages Of Technic Civilization

Poul Anderson wrote one short story, "The Star Plunderer," set during the Time of Troubles between the Solar Commonwealth and the Terran Empire and another, "A Tragedy of Errors," set during the Long Night between the Terran Empire and the Allied Planets.

The Long Night is the Troubles writ large, a second social breakdown but on a vaster interstellar scale. During the Troubles, the Commonwealth government collapses while the Baldic League, uniting alien Gorzuni with human barbarians, occupies the outer Solar System and raids Earth for slaves. An entire series could have been set during the Troubles which end when Manuel Argos leads a slave revolt, attacks Gorzun and founds the Terran Empire, whose Fall a thousand years later precipitates the Long Night.

We know less about the Long Night, just that, throughout the four hundred light year diameter sphere that had been the Empire, Imperial rule ceases and interstellar travel is significantly reduced although there is no reason why some isolated planets cannot have retained their own technologies and civilizations. Sandra Miesel's Chronology of Technic Civilization tells us that, in the middle of the fourth millennium:

"War, piracy, economic collapse, and isolation devastate countless worlds."
Sandra Miesel, "Chronology of Technic Civilization" IN Poul Anderson, Rise Of The Terran Empire (New York, 2011), pp. 663-672 AT p. 671.

Countless worlds but not necessarily all. "A Tragedy of Errors" features a potential series character, Roan Tom, who gains economic power on the colony planet Kraken, which later joins the Allied Planets.

A notable feature of the Long Night is that the former Imperial volume of space is not, as expected, occupied by the rival Roidhunate of Merseia. It seems that "The Empire and Merseia wear each other out." (ibid.) We see the beginning of Merseian demoralization at the end of The Game Of Empire, the last work set before the Long Night.