This is the first in the classic Andre Norton Fantasy series, and it sets up the series pretty well. Simon Tregarth is a soldier who got into trouble with the army because of black market activities, and is now being chased by some criminals who are trying to kill him. He knows his time is running out when he is offered an escape via the Siege Perilous, which transports him to another time and space, Witch World. There he joins a faction that is fighting against another group that seems to be somehow getting assistance from another world as well. Are they getting this from Nazis from Earth? I don't think it is ever clear, but they are definitely baddies.
Paperback, 293 pages
Published July 1, 1995 by Warner Books.