The planet was rich--at least before they brought in the mercenaries--and the disagreement over a puny few percentage points in loading fees at the local spaceport. But that was enough to let the locals invite in mercenary soldiers including Hammer's Slammers. And once the Slammers are in a conflict, they follow through. How they follow through may not be pretty--and may not be what the governments who invited them in intended, but they follow the money and their contracts, not some abstract ideal of good. After all, it wasn't them who started the war--they simply intend to end it.
PAYING THE PIPER follows the path of Lieutenant Arne Huber from initial landing on Plattner's World to the war's wrapup. Getting there requires Huber to fight across the planet, facing a variety of local militias (hardly worth fighting), and experienced mercenary companies. Sometimes, though, it seems like the enemy is within the nations that hired him. And sometimes, it even seems that the white mice of Hammer's police and intelligence group is the real enemy. But it's up to Huber to find a balance that leaves him loyal to contract, his men, and his unit (although not necessarily in that order).
Author David Drake doesn't delve especially deep into Huber's character, but he does give Huber enough detail to make him sympathetic and interesting to the reader. And Drake's analysis of the military is largely small unit and tactical rather than strategic. But Drake keeps the action rolling, sending Huber from one deadly firefight to the next with virtually no respite.
Although Huber and Hammer's Slammers are the heros and 'goodguys' of the story, Drake doesn't attempt to romanticize war or the military. Most of Huber's men and women are killers, pure and simple. In many cases, they kill when they really don't have to and when the killing serves no useful military purpose. Worse, Hammer command doesn't especially care whether it is supporting properly elected officials or helping those elected through voter fraud. They're there to get the job done. For me, this darker approach to the military future is interesting and timely. By the time the battle is over, everyone is a loser. And Drake doesn't even attempt to persuade the reader that all of this killing has been for a noble purpose.
If you're looking for an exciting military action SF adventure, with just a touch of cynical realism, it's hard to go wrong with PAYING THE PIPER.