Soldiers by John Dalmas

After centuries of horrible high-tech wars, galactic humanity had at last emerged from The Time of Troubles, firm in the belief that nothing was preferable to war, convinced indeed that to believe otherwise was psychotic. But soon the Commonwealth of Worlds is going to need every "psychotic" it can get its hands on—because trouble is heading straight at them in the form of more than 14,000 warships from somewhere.

Every inhabited planet that armada encounters is wiped clean of human life to make room for alien colonists. Either humans will again learn how to be soldiers, and quickly, or humanity will join the dinosaurs as an interesting extinct species.
Genre: Science Fiction