I think this would be a mistake, it would give the Russian Army motivation to fight that has been lacking so far.
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The Nazis steam rolled through Ukraine in 9 weeks, couldn’t take Russia, you think the worn down Ukies are stupid enough to cross into Russia? They have not, and will not. Once energized, Russia would fully mobilize ( they have not yet), then Vlad would crush the Ukies with full support of the Russian people- full mobilization.
Your nonsense aside- the Ukies are being strangled by lack of fuel, arms, equipment, rail lines east destroyed, fuel from Romania stopped, when Vlad decides to destroy the dozen or so bridges across the Dniepner- all over for the Ukies. Vlad has exercised much restraint, restraint that will go out the window if the Ukies crossed the border in a publicity maneuver.
Glad you brought in some history since that seems to be ignored. The Red Army initially did very poorly against the Wehrmacht. They finally overwhelmed the Germans with numbers, masses of equipment, and commanders who arose from merit rather than politics. They absorbed mass casualties in a war of attrition the Nazis couldn’t match. Russians are not as zealous as they were then, but the Ukrainians should not interpret doing well for a couple of months to mean the Russian can’t overwhelm them eventually. The Russians likely will eventually declare war, mobilize and become more competent; Ukraine would do well to delay as long as possible.
The Ukes welcomed the Nazis as liberators. They remembered the government induced famine. When the Nazis started their ethnic cleansing in the east is when the Ukes turned against them