Kingdom River

Volume 2 of the Snowfall trilogy. A slight change in Jupiter's orbit has brought an ice age to Earth. 600 years from now, mankind has regressed technologically to a pre-gunpowder level.

A Mongolian horde has crossed the frozen Bering strait and is overrunning North America. A small army from northern Mexico and the powerful but rigid Middle Kingdom must unite to stop them. Meanwhile, New England has achieved disturbing results in genetic engineering despite the rest of the world's technological decay.

Smith's world is brutal, a point he drives home again and again. What we would consider to be good, socially conscious people either compromise their principles repeatedly--at great personal cost--or else are destroyed.

Wonderfully written. Don't get attached to any of the characters in this series...Smith is almost George R R Martinesque in his willingness to dispose of them.