This is book number 1 in the Liaden Universe Books of Before series.
Centuries in the past, mankind fought a seemingly unbeatable adversary from sector to sector across the Spiral Arm until the war ground to a standstill and the Enemy withdrew. Believing that they had won, the citizens of the galaxy rebuilt. The Inner Worlds, which had escaped the worst of the war's ravages, became even more insular, while the Rim worlds adopted a free and easy way with law and order. Now, hundreds of years after their withdrawal, the Enemy is back--and this time they'll be satisfied with nothing less than the extinction of the galaxy.