Was Hiroshima the biggest event of World War 2?

Transcript
Extracts from an American report on the possible impact of atomic weapons on US cities

Catalogue ref: FO 371/59640

The overwhelming bulk of the buildings in American cities would not stand up against an atomic bomb bursting a mile or a mile and a half from them.

And the people? We must not too readily discount the casualty rate because of the teeming populations of congested Japanese cities. American cities too have their crowded slums, and in addition tend to build vertically so that the density of the population is high in a given area even though each apartment dweller may have more living space than his Japanese equivalent.
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The casualty rates at Hiroshima and Nagasaki, applied to the massed inhabitants of Manhattan, Brooklyn, and the Bronx, yield a grim conclusion. These casualty rates, it must never be forgotten, result from the first atomic bombs to be used and from bombs burst at considerable distances above the ground. Improved bomb, perhaps detonated more effectively, may well prove still more deadly.