Through various raids and acts of sabotage during WWII, the Allies succeeded in preventing Germany from coming into possession of large quantities of heavy water produced by the Norsk Hydro plant in Vemork Norway. These actions were carried out at considerable expense in terms of Allied troops and civilian casualties, presumably because the heavy water was perceived as a providing Germany with a substantial "leg up" in producing an atomic bomb.
But from my (admittedly limited) knowledge of the construction of Fat Man and Little Boy, heavy water was not a factor (at least not in the final weapons). Why was it believed that heavy water would make a German atomic bomb much more likely?
(The reason I ask is that I recently saw an old (1965) movie ("The Heroes of Telemark") about the sabotage operations. While a bit too "Hollywood" (with Kirk Douglas and Richard Harris), it was apparently reasonably accurate. And several years back I saw something on PBS about the sinking of the ferry carrying the heavy water.)