The available facts about the power of the atomic bomb as a military weapon lie in the story of what it did at Hiroshima and Nagasaki.
One further measure of safety must accompany the others. To avoid destruction, the surest way is to avoid war. This was the Survey’s recommendation after viewing the rubble of German cities, and it holds equally true whether one remembers the ashes of Hiroshima or considers the vulnerability of American cities.
Our national policy has consistently had as one of its basic principles the maintenance of peace. Based on our ideals of justice and of peaceful development of our resources, this disinterested policy has been reinforced by our clear lack of anything to gain from war–even in victory. No more forceful arguments for peace and for the international machinery of peace than the sight of the devastation of Hiroshima and Nagasaki have ever been devised. As the developer and exploiter of this ominous weapon, our nation has a responsibility, which no American should shirk, to lead in establishing and implementing the international guarantees and controls which will prevent its future use.

U.S. Strategic Bombing Survey: The Effects of the Atomic Bombings of Hiroshima and Nagasaki – June 1946 (PDF)