We turned a ton of cities into ashes during WWII without the help of nuclear weapons (except at the very end).
https://en.wikipedia.org/wiki/TNT_equivalent
* The total energy of all explosives used in World War Two (including the Hiroshima and Nagasaki bombs) is estimated to have been three megatons of TNT
* The total global nuclear arsenal is about 30,000 nuclear warheads with a destructive capacity of 7,000 megatons or 7 gigatons (7,000 million tons) of TNT
And TFA talked about a nuclear winter from burning cities when using a hundred Hiroshima bombs, not the US/Russian arsenal. We ought to have learned about those effects, just from WW II.