Pharaoh

Benoni Strang's workers on Babur have a secret base on a Baburite moon. The Baburites' oxygen-breathing mercenary army get a whole planet which they know as Pharaoh.

About a million human, Gorzuni, Merseian and Donarrian mercenaries occupy Hermes with a comparable number held in reserve. Recruited in many places on dozens of planets, either they did not register on anyone's statistics or, if some League company did notice, it did not inform anyone else.

"Space is too big, and we too divided."
-Poul Anderson, Rise Of The Terran Empire (New York, 2011), p. 220.

Isolated for years of training and preparation on Pharaoh, the soldiers have accumulating pay, beer halls, brothels, a multi-sensory library and a hot, wet, barely terrestroid, permanently clouded, planetary environment to explore. The clouds prevent them from learning where Pharaoh is. Falkayn thinks that it is more likely to be inside known space. Outside, it could be discovered by explorers, who continually expand known space. Inside, it might already have been explored once but dismissed as of no interest.

Learning that they are working for Babur against the Solar Commonwealth does not demoralize highly disciplined mercs who were recruited precisely because they were already alienated from Technic civilization.