[spoilers] In the first book the earth is attacked by Von Neumann probes launched preemptively by a civilization that wants to destroy potential agressors befo

[spoilers] In the first book the earth is attacked by Von Neumann probes launched preemptively by a civilization that wants to destroy potential agressors before they develop the technology to become a risk. The "benefactor" civilization group organises rescue and counter-assaults as a means of pan-civilisation trust building. The second book looks at the ethics of destroying a civ based on decisions it may have taken in its distant past. Anvil has something of a young adult style, but the game theory and strategy elements are quite satisfying I think.