A Resistance of Witches by Morgan Ryan
In this historical WWII fantasy, a British witch joins the war effort to help track down a dangerous magical relic before a coven of Nazi witches find it and bring it to Hitler. After a Nazi witch infiltrates the Royal Academy of Witches with devastating consequences, Lydia Polk wants to join the fight, but meets resistance from the elder members of her coven, who do not want to sacrifice any more lives in the service of a government that has forced them into hiding for generations.