I recently came across a question posed by another writer asking for advice on writing a fantasy-based WWII novel. He voiced the concern of whether or not it’s been done and well received, and his second concern was how to write such a story without treating certain historical and personal elements in a flippant manner.
To address the first concern, there are definitely popular fantasy stories that use WWII as a setting. It’s no secret that Hitler and the SS were into occultism and the mythology of a god-like race. They sought mystical power in addition to temporal might. This would definitely be an interesting basis for a fiction fantasy–and, there are several that have already been written. Among them include Robert McCammon’s The Wolf’s Hour and Mike Mignola’s Hellboy comics. One of the most recent (which happens to also be on my TBR list) is Bitter Seeds by Ian Tregillis. So WWII fantasy can be done, and can be great reads!