Thumpalumpacus
Major
Is invading Ukraine and destroying their own economy the worst miscalculation in Russia's history? Others could be provoking war with Japan in 1904, or war with Germany in 1914, or the 1939 Molotov–Ribbentrop Pact, or purging its officer corps just before Barbarossa in 1941? Any others? Had cooling heads in London not prevailed, Russia's attacking of British shipping in 1904 might have made the list.
You might also add the Afghanistan war. And as I've written much earlier in this thread. I suspect that this too might be a quagmire that brings down a regime.
I definitely think that this is Putin's worst, and possibly fatal, miscalculation. If he backs away, he alienates the hard-liners; if he stays in nose-deep, the general polity may well pull out support, as the economy is suffering inflation and deprivation.
Both the Tsars and the Politburo learnt the hard way that even a dictatorship must pay attention to the will of the people.