By June 1944, Germany had little hope of winning anything. The Generals wanted to sue for peace, but The High Command as well as the Allies wouldn't have it. Hitler knew he couldn't end the war based on what was going on with the Attrocities that were transpiring. The Allies wanted Unconditional Surrender.
D-Day was necessary to end the war and if it failed the war would have lasted much longer with higher costs in lives. But Germany was finished, Russia and Allied Bombing saw to that. Bombing not because of destruction, production actually increased until the very end, but because it exhausted the Luftwaffe and gave the Allies Air superiority.
As far as the Bomb...The Germans scientists involved, after the war, claimed they weren't trying to succeed on principle, but final analysis shows they weren't even close and going in the wrong direction.
As far as Truman dropping the Bomb on Europe...Germans were too much like us and too close for something of that magnitude, remember Japan was considered a lot different and that they would not surrender at any cost AND halfway around the world.
Besides, the Bomb was more a message to Communist Russia than it was a strategic weapon.
Monrad is right about the Vengence weapons though. They were the first ballistic missiles and a priority to silence.