Birds of prey
A lot of people don't realize that David Drake studies history and that he uses events that happened in the past as plots for many of his outstanding novels. In this masterful tale, it's all in the past with a couple of additions from our far future, blended together as only he can. Take an agent for the Roman Empire who is usually given the tough jobs which require a tough man to deal with them. Add a person from the far future out to save humanity all down the time line, add some detestable bug like creatures out to swarm and destroy mankind. Throw in a few companions, pirates, dense clerical officials, traitors and a whole lot of action and you have a tale worth your time reading.