Adiamante

Earth was once the capital of an interstellar empire, but now it's just another inhabited planet in a forgotten universal order. Its citizens survive through kindness, environmental conservation and non-aggression. But humanity's great strides toward erasing its violent past are threatened when 12 ships made of indestructible adiamante appear in orbit. The ships are from a former colony bent on subjugating the Earth and its inhabitants. But the strict ethics that have helped revitalize the Earth may be its downfall, as the human homeworld struggles to find a way to save itself without violating its new-found principles.