Agent of the Terran Empire. A Flandry Book

"If it pleased Ruethen of the Long Hand to give a feast and ball at the Crystal Moon for his enemies. He knew they must come. Pride of race had slipped from Terra, while the need to appear well-bred and sophisticated had waxed correspondingly. The fact that spaceships prowled and fought, fifty light-years beyond Antares, made it all the more impossible a gaucherie to refuse an invitation from the Mersian representative.
Besides, one could feel delightfully wicked and ever so delicately in danger."It is the common fate of empires to grow old and jaded: Rome, Byzantium, Britain, America, and so on to the Empire of Terra itself, each has near the end succumbed to the same weary "sophistication" that allows a warlord of Merseia to make a mock of a race whose star-conquering ancestors found the Merseians a race of pre-technic barbarians huddled in stone piles - and saved them from extinction.
Flandry himself has come to understand that there is no more point to all his victories than that a few trillion of his fellow creatures may live out their lives before the coming of the Long Night of galactic barbarism. That he will not have shortened that coming Dark Age one bit - only postponed it. That the barbarians always win in the end, and are always followed by a new round of civilisation.