The Trigon Disunity Series by Michael P. Kube-McDowell

The devastating Food and Fuel Wars have turned once-powerful nations into isolated farming communities. Barter has replaced currency, and scientist - considered responsible for the world's misery, are burned at the stake. Hidden in the Idaho hills, astronomer Allen Chandliss secretly monitors his radio telescope, listening for signs of intelligent life, hoping that aliens will come and improve things on Earth. For seventeen years he has waited patiently. His patience is about to pay off...

Merritt Thackery knew from boyhood that his only dream was to be part of the Unified Space Survey. He prepared himself with five years of training on an exquisitely boring space tug, and with a speciality in alien linguistics. When his chance finally came, he was more than ready. But even Thackery's boyhood dreams could not have prepared him for his final mission, beyond the borders of known space: To seek out an unknown enemy and win Earth's freedom at last!

"The work of an alert intellect dealing with major issues", says Locus of the conclusion to the "Trigon Disunity" trilogy. In Empery, a newgalactic empire has coalesced some 60,000 years after the destruction by the alien Mizari. The Unified Worlds, despite eleborate defenses, are devided over the development of a world-wrecking new weapon while the Mizari threat intensifies.