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Putin wants to destroy Ukraine: he has always want to do that.
It would explain perhaps why, having actually - despite what some claim - achieved one apparent aim of occupying territory in SE Ukraine all the way to Crimea, he did not try to halt it there and declare "peace".
It's why they must be driven from the terroritory of Ukraine asap.
The dam was a contingency they had up their sleeve for months and the decision to blow it was probably a political one, as it has apparently hit their own military quite hard (it also means that upstream, the falling water levels on the Dnipro will make it easier for the Ukrainians to cross (!). There is also the contingency of the Enerhodar NPP, whoch has also apparently been mined, although it is unclear exacyly what the consequences would be if that was destroyed. Not good certainly, but it would have environmental repercussions for Russia too.