The USA engaged in much diplomacy to end World War II, but not with its opponents. The only thing acceptable to USA from its opponents was total surrender without conditions. This stance by USA probably had the effect of prolonging the war, since no one wants to surrender without retaining some rights. So in this respect you could say that lack of diplomacy with enemies had an effect on the ending of WW II. It may have postponed the ending.
With Britain, on the other hand, USA engaged in much ongoing diplomacy. Much discussion and arguing and compromise as to how to pursue victory in Europe, took place between war-leaders of Britain and USA, and at Tehran, Russia. This finally led to a blueprint for smashing Germany, in which each of the three played big parts in ending World War II and establishing the terms of peace.