The bomb dropping was actually a US reaction to the success of the Russians in meeting their objectives in the Allied War. Russia had already marched into Berlin by this time, and their armies were poised to invade Japan. Not wishing this to happen, due to the post-war geopolitical implications of such a thing, the US administration hastened to use the atomic bomb.
Development and use of the atomic weapon was also a fantastic way to place the US in a powerful economic and political position - a position from which they could steer world events for the next century.
Incidentally, by the time Hitler had sent troops to attack Stalingrad, the German attempt at building the nuclear weapon had collapsed, mainly due to a lack of resources. It is suspected that some of their top scientists also played a role in deliberately delaying the development of the weapon. The U.S. continued development, knowing that the atomic bomb would yield supremacy in world affairs after the war.