For some strange reason folks just don’t give 400kt nuclear warheads the respect they deserve. It would not take to many well placed 400kt warheads to send us, the USA, back to the bronze age for a generation or two.
Yes, a 400kt over a city would be a nightmare that can’t be compared to. Even a 100kt. It would be ‘the end of the world’ for 50 miles or more. The fallout would be ‘end of the world’ for for 500 miles as well.