In 1961, a US B-52 carrying two 25 MT hydrogen bombs accidentally released them near Goldsboro NC. One buried itself in a hillside when its parachute failed to open, while the other one gently glided down to earth. When the one with the failed parachute was recovered, it was discovered that five of its six safeties had failed. A 25 MT bomb was saved by precisely one safety not going off. Imagine if the United States had dropped a 25 MT bomb on itself in 1958...what sort of effects that would have on our national psyche, nuclear disarmament in general, politics, etc!
But to carry this a little further, I want to change the locale. B-52s were stationed at Carswell AFB, within Fort Worth, TX city limits. Imagine the same accident happening over downtown Fort Worth now. One day in July 1961, lets make it July 4, a B-52 has an accident, and several seconds later a 25 MT device is detonated at ground level in Fort Worth, TX.
I've done some initial calculations for this. It is likely that a ground burst over Fort Worth will kill 600,000 Americans within minutes, more than died in both World Wars combined. Buildings in Dallas will have shattered glass, but not too much other damage. Of course, this is before the fallout of a groundburst starts dispersing itself across Texas and wherever the wind may take it.
What do you think the effects of such a disaster would be?