We now know that Ukrainians mean to fight the Russian invasion with determination and bravery. In that fight they must have support. We cannot fail to admire fellow Europeans as they take up weapons to defend their country against meaningless aggression. But what does support mean? The British government disagrees with many Ukrainians that it shares the same threat and should join the same fight. With Vladimir Putin in his possibly deranged state, the resulting paranoia and slaughter would play into his hands. It would be truly appalling and, on present evidence, risk a catastrophe that is as uncertain as it is unjustified. Britain has rightly joined the rest of Europe in expressing its horror. But when the British government was called on to do more than signal virtue by admitting possibly thousands of Ukrainian refugees, the Home Office reverted to type. Boris Johnson announced that relaxed visa rules would only apply to immediate family members and the immigration minister, Kevin Foster, suggested others could perhaps apply for visas as fruit-pickers. Though he later deleted this cynical tweet, it showed that when push comes to shove Whitehall’s “hostile environment” is instinctive and its support for Ukraine all but hollow. Otherwise there are economic sanctions, the capitalist equivalent of medieval siege and famine. Their severity has been impressive, showing Britain united with Europe and the US against evil. The trouble is that, unlike famine, history shows almost no sign of sanctions achieving their desired aim. The present measures will, as usual, hurt the poor rather than the rich, and benefit such bystanders as Chinese tycoons and the gas and oil giants. The edifice of dictatorship is largely immune to the subtleties of markets, exchanges and visa bans. Heavily sanctioned nations, including Iran, North Korea, Cuba and Syria, currently show little sign of regime change. If anything, sanctions entrench power. This is Putin’s war, not Russia’s, but how can Russia speak? The justification for sanctioning its economy, banks, travel, sports teams and cosmopolitan diaspora is that this will somehow persuade Putin of the error of his ways, that he has made a terrible misjudgment. I have seen no account of how this is supposed to happen. Do we intend for Russians to suddenly do what they have not done for a century and rebel and topple him? Will a Claus von Stauffenberg-style conspiracy storm the Kremlin, or a posse of cronies cart him off to an asylum? As for how long this will take, the west – and especially Britain – should know how hard it is for a proud regime to withdraw its unwarranted aggression against a supposedly weak state, and how vacuous its excuses become. In Putin’s pathetic explanation of his Ukraine invasion at the weekend, I heard echoes of the west’s war in Afghanistan, when we were told that the British army was killing Pashtuns in Helmand to keep people safe on the streets of Britain. It took Britain years to admit defeat and leave. Is Ukraine – and Russia – to wait that long for Putin to die or fall off his perch? Realpolitik says that somehow Putin must be induced to withdraw his armies and admit that his “buffer-zone” strategy has failed. Mediation must find some form of words to cover his retreat, however painful those words may be for some to swallow. Ukraine and Russia must live as neighbours once more, as geography has required them to do throughout history. The alternative is for Kyiv to suffer years of occupation and puppet government, until liberal voices within Russia persuade whoever occupies the Kremlin to relent. Those voices are what everyone – Ukraine, Russia and the rest of Europe – most desperately need just now. They will require not ostracism and hostility from the west but contact, friendship and encouragement. Someday, preferably soon, the signalling has to stop and serious diplomacy resume. Simon Jenkins is a Guardian columnist Join a panel of journalists, hosted by Michael Safi, for a livestreamed event on the Russia-Ukraine crisis on Thursday 3 March, at 8pm GMT | 9pm CET | 12pm PST | 3pm EST. Book tickets here