Trapped in Time (Star Trek: Deep Space Nine #12) by Ted Pederson

A blast to the past! Jake Sisko is wondering where life will take him next... and the answer is the past!

Chief O'Brien has promised to deliver a gift to a physicist, Professor Jonathan Vance. And Jake and Nog are allowed to accompany him to France. Vance shows them his "time machine", the first one ever to control time jumps with accuracy. When his assistant, Kruger, attacks him and steals the control device, Jake, Nog and O'Brien leap through the portal after him. They find themselves in Normandy, France, in 1944 during the middle of World War II. With Kruger joining the ranks of the Third Reich as a colonel, history is about to be changed forever. Can they stop Kruger from informing Hitler that a secret invasion will happen in Normandy?