In at the death - Harry Turtledove

In the fourth volume of the Settling Accounts sequence it is
1945, and the war between the northern and southern American states that
began in 1861 ends at last, as the most inventive and enthralling
alternate-history series of our time comes to its surprising conclusion.

Victory seems assured as the United States of America drive deep into Confederate States territory, uncovering shocking death camps as they go.
But both sides in this war have been working on unimaginably powerful new weapons.
Can a new kind of bomb save the Confederate States from defeat at the eleventh hour?