Rulers of Darkness by Harry Turtledove

The author of the Worldwar and Great War series displays his virtuoso command of the details of WWII in this fourth book (after 2001's Through the Darkness) about a conflict between mythical feudal kingdoms using magic instead of science as the basis for technology. Aficionados will enjoy picking out the parallels the Japan-analog Gyonghos Empire, for example, fights "the grinning dwarves of Kuusamo" (i.e., the United States).