The mushroom cloud over Nagasaki as photographed by Charles Levy from one of the B-29 Superfortresses used in the attack. The necessity of the bomb is hotly debated by historians (click on the photo for more details). But as author Martin J. Sherwin notes, by dropping it, “the United States signaled to the world that it considered nuclear weapons to be legitimate weapons of war.” And that’s a responsibilty we now have to live with … or die with.