Home Before the Sky Falls (1993)

From the back cover:

Bonparr, secure in its legacy of military power and just rule, has alarming news. Refugees are streaming eastward, driven forward by an apparently unstoppable war machine which is attacking and destrying everything in its path. Perhaps the army of Bonparr can stop it, but not without help.

"Volunteers" are needed, especially those with experience at dealing with ancient devices and who can supply their own heavy weapons. In short, Bonparr needs mercenary adventurers, and it will do almost anything to get them. Everyone knows you can't fight Bonparr... but can you survive fighting for Bonparr?