Union Soldiers transported to new world?

[MilesyUK]

I was wondering if anyone knew what this series of books may be.
Basically, a regiment of Union soldiers are travelling on a boat and go through a fog bank and when they emerge on the other side, they find themselves in a new world, where humans are subservient to a race of monsters. They then go about helping the humans try to free themselves.

It was a series of books and I never found them all, but would like to. If anyone could help I would be very grateful.

Many thanks in advance.