Confederation vendre by Martell, Eric NEUF

The origin of the confederacy of the three races.

After aliens attack the Earth, the surviving humans exist in a post-apocalyptic world and are divided into groups that contest power.

One group has allied with two alien races. The disparate races have united in using technology captured from the original alien invaders.

Seemingly impossible tasks face this fledgling confederation: They must destroy the threat still posed by the original invaders while being involved in a conflict with barbaric remnants of the Federal government for control of the remaining population of the US. To complicate matters, they must simultaneously find a way to strengthen their working relationship, develop trade protocols and a mechanism for mutual defense in the face of potential incursions of unknown hostiles from outside their sphere of occupied planets.

This is the third story in the Gaea Ascendant Trilogy