Stepsons of Terra

From back cover: For 500 years the distant Terran colony of Corwin had no communication with Earth. But now the invincible war-horde of the Klodni was on the march and the long-forgotten planet desperately needed Earth's help. Baird Ewing was appointed ambassador, sent to get that help.

But Earth. . . Earth had degenerated into a decadent world of worthless pleasure seekers, and would never be able to give the aid that would save Corwin. Earth had nothing to offer, and Ewing had so little time -- the answer was right under his nose if he would only look. . . .