I don’t think that Putin has a mustache, and the laws of physics clearly state that a mustache must be twirled while making such a statement.
But, in reality, no, we cannot accept that. If he gets away with that, then next week, there goes Akron, and then Galveston the week after that. He would simply keep attacking us, as we demonstrated that we will just accept it and not retaliate.
In that particular scenario, which I find fairly unlikely, I would think that the best response would be, “You have attacked us with a nuclear weapon, destroying our city and killing our people. We are launching one nuclear weapon at one of your smaller cities, and will inflict similar damage to you. We have no choice in this matter, as to do other will invite you to continue your attacks on our country. If you choose to continue your attack on our country and to escalate it, so will we, and we will all lose.”
As to the actual matter of reality, we also cannot ignore if they nuke a single city of Ukraine. If we do, then that means that it is now permissible to use nukes in war. Putin won’t settle for one nuke on Kyiv, he’ll follow with a few more, and target other cities as well.
And maybe we retaliate in kind and proportion, involving third parties in this. He nukes Kyiv, we nuke Minsk. He nukes Lviv, we nuke Mogilev. We start nuking his allies when he nukes ours, and he may not have many friends anymore.
I don’t see that as the response we go with, but it should be. Once nukes get on the table, there are no longer any humane responses left.