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.
Diplomatic efforts really had very little to do with ending World War II and setting the terms of the peace that was to follow.
The only way in which diplomacy came into play was in talks between the Allies. In talks such as those at the Yalta Conference, the Allies decided what would be done with various defeated territories after the war.
When it came to Germany and Japan, however, the Allies demanded unconditional surrender from both countries. These surrenders were gotten by military force -- in the case of Japan, by the use of atomic weapons -- and not by diplomacy.
I do not think that diplomatic efforts had much success in ending the World War II. Two main countries of the Axis countries held out firmly till the defeat was staring starkly in their faces. Hitler committed suicide in face of imminent capture, and the leaders after him quickly surrendered as they had already realized that they cannot win the war. Similarly<Japan was also forced to surrender by the show of strength by way of dropping of atom bomb in Hiroshima and Nagasaki. This development made it amply clear to Japan that they have no chance of winning the war, and they also surrendered.
However I believe diplomacy played a great part in securing a lasting peace after the war.
Diplomacy had very little to do with ending World War II. Instead there were only negotiations between the Allied powers while the only negotiations extended to the Axis powers were unconditional surrender. The main goals of the diplomacy during this period were to plan postwar arrangements and peace. (at the Atlantic Charter and Yalta Conference). However even these efforts were futile in establishing peace because their idealistic clauses would intrude on the nations’ security and economic interests. The biggest success of the diplomatic efforts was the negotiation of the terms to enter into the future United Nations.
The only diplomatic talks were between the Allies. Such talks were the Atlantic Charter and the Yalta Conference. When talking to Japan and German, the Allied powers demanded unconditional surrender. The Yalta Conference was the meeting of FDR, Churchill, and Stalin in 1945 to plan the final stages of the war and postwar arrangements. The Atlantic Charter was pretty much the same thing. After the Yalta Conference, FDR announced that it had been a “great success”.