Adiamante - L. E. Modesitt Jr.

In the far future, Earth is just one planet inhabited by humanity, the capital of a long-abandoned interstellar empire. The people of Earth have built a new society and slowly the planet is recovering from millennia of selfish exploitation and the destruction of wars.

Suddenly, a former colony's fleet of twelve warships built of nearly indestructible adiamante appears in orbit and tries to intimidate the people of Earth into submission. The people of Earth will not surrender, but their principles also don't allow them to take defensive measures until the fleet actually attacks.

Ecktor deJanes is the newly appointed planetary coordinator and has the terrible responsibility of protecting the lives of all the Earth's inhabitants. Somehow he must maintain his society's principles while preventing the fleet from turning the planet into a lifeless ball of rock.