> Wait, "it takes two" implies some consent of both parties.
Yes. In this case, both parties consent to threatening each other with their nuclear arsenals. They consent to gradually raising the stakes in this poker game, and eventually calling each other's bluff. It may very well turn out that one of the two is bluffing. Or it may not. I really don't care all that much to find out which.
> If someone is accosted we don't say they--as a victim--share half the fault.
> ... trespassing and destruction justified?
> ... a blank check to annex neighbors?
We are speaking different languages here. You are talking the language of ethics and apportioning the blame. I am talking the language of survival, which I think the article in the title post talks as well.
If - following your metaphor — you live next door to a neighbor who has hired a bodyguard, and in the moment of intrusion the bodyguard and the intruder start firing at each other from a rocket launcher, explode a cistern full of gas, and as a result the intruder dies, the bodyguard dies, your neighbor dies, and you die — would you particularly care in those last terrifying moments whether it was the intruder's fault, the bodyguard's fault, or your neighbor's fault? I know I wouldn't.
> Said another way, does Putin having nukes constitute a blank check to annex neighbors?
Both no and yes, depending on which way you turn the question. It doesn't give him the right to do so, but it sure gives him the means.