"The Star Plunderer"

Poul Anderson's "The Star Plunderer":

was published in 1952, four years before the first Nicholas van Rijn story;

mentions a Commonwealth within the Solar System but does not use the phrase, "Solar Commonwealth";

makes no mention of the Polesotechnic League;

has Manuel Argos proclaiming an Empire that he calls the Empire of Sol, not the Terran Empire.

When Poul Anderson decided that Nicholas van Rijn, and therefore also the Polesotechnic League, had preceded Dominic Flandry and the Terran Empire, then "The Star Plunderer" became the turning point. Its Commonwealth had been the Solar Commonwealth. The barbarians who sacked Earth and enslaved Terrestrials had been armed by unscrupulous companies of the Polesotechnic League. The dissolution of the League had preceded the attacks on Earth. It is difficult to accept that the sophisticated interstellar civilization of Mirkheim was succeeded by the pulp magazine barbarism of the four-armed Gorzuni and their rebellious human slave, Manuel Argos - but Anderson converted his League and Flandry series into a single future history series and it works.