The Doomfarers of Coramonde (Coramonde duology)

Just yesterday Sergeant Gil McDonald and his APC crew had been fighting their way out of an ambush in a Viet Nam jungle. In the middle of the enchange of fire some kind of magic had transported them to this Fantasy Land complete with flying dragons, wizards, crazy castles, and dispossessed princesses.

They would stay trapped here forever unless they could rescue the sorceress Gabrielle. To reach her, Gil and his men would have to infiltrate Hell itself!