How do i find the name of a trilogy of fantasy american civil war books that i read in the 1980's?

Invilved a platoon of soldiers in an ironclad ship, travelling through time to another dimension.and using their advanced weaponry to defeat the enemy of a local town. After this they discover that the other tribe were motivated by their fear of 10 feet tall "monsters" on horseback who use humans as cattle for food and menial tasks. I think the author was female, but I'm not sure of the date of the books