This is book 3 of the "Manticore Ascendant" series.

This note contains spoilers for the book.

By David Weber and Timothy Zahn, with Thomas Pope.

Still back before Manticore knows about their wormhole junction (but the Axelrod corporation knows). They've hired mercenaries to go take the system, with cover from the Grand Duchy of wossname (which is far away), follow-on from the Battle of Manticore in a previous book.

We see the beginnings of the process of building a professional navy, Manticore getting the first inkling that foreign relations may be important, and friendly cooperation with the Republic of Haven.

Axelrod doesn't get to take the system, of course. We also see some messy internal politics around the succession in Manticore.