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You've admitted that wiping out a city because someone tried to kill you is a disproportionate response. What you don't seem to understand is that wiping out civilians en masse is always a disproportionate response.
The only exception to the plain fact that a nuclear assault is a war crime is if you target a missile facility with tactical nukes out of necessity, and a small number of civilian deaths are unavoidable.
Targeting a city with strategic nukes is not allowed, regardless of the attacks you have suffered---yes, even if you are only trying to exact revenge. The "laws" of proportionality don't allow you to murder millions of innocents just because your enemy murdered millions of your innocents. That's just obvious. They do allow you to use WMDs if your enemy did, but they don't allow their blatant abuse.
(That's why the US developed more precise, smaller missiles/warheads, so they could target silos closer to population centers than the Russians could.)