The breach of the Nova Kakhovka dam is most worrying for what it says about the mind of Russian President Vladimir Putin and his generals – and their capacity for dangerous escalation.
It takes the war in an even more perilous direction.
The military impact is likely to be temporary. Armies blow dams or use them to unleash flood waters for tactical advantage.
The Soviets and the Germans both did it in World War Two. The Ukrainians did it during this one.
But the gains generally do not hold. Water drains away, the ground dries out.
Warning of ‘grave consequences’ after dam blast – live updates
Ben Barry, a land war senior fellow at the International Institute of Strategic Studies, said: “It could set back any assault river crossing for a couple of weeks.
“Difficult to tell for how long. But only once the water subsides and the ground dries out will Ukraine have the same chance of a river crossing as it did before the flood.”
And he believes an attack across the swollen Dnipro is not out of the question even now.
“It’s not impossible to do an assault river crossing across a river that’s in full flood. It’s just more difficult,” he said.
The Russians have blamed the Ukrainians for the attack, but most analysts have dismissed that as unlikely to impossible.
The Russians have a proven track record for accusing the other side of doing what they have themselves done. And the Russians have most to gain. Up to a point.
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The breach benefits the Russians by reducing the length of the frontline it has to defend and allowing it to focus attention in the east, but not indefinitely.
And it has blowback for the Russians too, flooding some of the defensive positions they have dug in on the southern bank since retreating there last August.
So temporary gain, some self-harm and all the opprobrium that comes with carrying out yet another war crime.
Where is the margin in that for Vladimir Putin? It looks rash and premature. A disproportionate and irrational act.
But that may be the point.
Russian president ‘excels in scare tactics’
In war, it can pay to do the crazy thing, to look unhinged and keep your enemy guessing at your next act of madness.
Putin excels in scare tactics and knows the dam blast makes him look more dangerous.
If Russia was irresponsible enough to blow the dam and unleash such destruction for limited advantage, what will it do next, planners in Kyiv and the West will be asking.
The fear now is for the Zaporizhzhia nuclear power plant. The dam breach endangers the supply of water to its cooling systems. Could Russia now sabotage the plant to change the course of the war?
The destruction of the dam undoubtedly changes the risk calculus in handling Russia, but correctly calibrating it will need cool heads so it is not overdone.
Putin has, after all, indulged in nuclear sabre-rattling for much of this war.
It has weighed on the minds of Ukraine’s allies and made them more timid in arming Kyiv.
But so far analysts say his nuclear bluster is just that.
The nuclear option
There is no sign of Putin starting the lengthy process of bringing tactical warheads out of storage and deploying them.
And any disaster at Zaporizhzhia threatens Russia most.
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The prevailing winds would be most likely to carry the fallout east across the Russian border.
The Russians have shown stunning disregard for the welfare of their own soldiers but a radioactive cloud over their defensive positions and logistics lines would be challenging to say the least.
What we can say for sure is this war has swung again in a more unpredictable direction and the longer it goes on, the more such lurches are likely to happen.