Cities of Mars
#3, Free Space trilogy
This book can be downloaded and read in iBooks on your Mac or iOS device.
Dale Kinmont recruits Martian volunteers to travel to Venus and aid in its defense. He assembles an army and dispatches them ahead of schedule. They'll beat Nat there with time to spare, and Dale is optimistic.
The plan is soon derailed. Nat and his army of convicts aren't going to Venus as planned--they're on their way to Mars. Dale's army's ship can't turn around, and they will never make it back in time.
He must find a way to defend his uncle's space station, as well as the Martian cities on the surface. He must do it without Mars's best fighting men, and he must do it quickly. Because Nat Leland is coming, and the most brutal men in the solar system are coming with him.