This is a fix-up of three short stories -- in rapid succession, developing the plot straightforwardly.
There was armed conflict over a planet called New Europe. The aliens -- the Aleriona -- claim an accidental conflct erupted into violence that killed all the colonists. But Gunnar Heim, having old ties to the planet, meets up with Endre Vadasz, who claims to have escaped and have proof that many of them are still alive.
No one will listen to him, or to Heim either, except France (most of the colonists having been French), but judicious exploitation of loopholes reveals that Heim can send forth a privateer, properly commissioned with letters of marque and reprisal. . . .
It involves an alien world where nuclear attacks aren't a big deal, scurvy, moving trees, a kidnapping, violent attacks by peace militants, the Aleriona not living in groups so much that Earth must give them separate residence or insult them, and much more.