Many of those leaders have now been knocked off balance by Mr. Putin’s scorched-earth campaign in Ukraine. Heedless of the Russian leader’s previous acts of murderous brutality — against Ukraine, Georgia and various Russian dissidents who crossed him — the current carnage has triggered a backpedaling stampede. For many Europeans in particular, the unfolding barbarity in Ukraine, alarmingly nearby, has placed nationalist parties and politicians in an unflattering and clarifying new light.

In France, ahead of national elections next month, right-wing politician Marine Le Pen has been embarrassed by a photograph of her shaking hands with Mr. Putin, featured in more than 1 million pamphlets recently printed by her National Rally party. Ms. Le Pen, who previously supported Mr. Putin’s annexation of Crimea, part of Ukraine, after her party took a 9 million-euro loan from a Russian bank to finance her 2017 presidential campaign, could plausibly have been regarded as a Russian asset by Moscow. As recently as last month, she parroted the Kremlin’s denials that Mr. Putin planned to invade Ukraine, saying she didn’t believe a bit of it. “I don’t see what … would be their interest there,” she declared.

Her eyes having been opened, she now asserts the invasion is “unjustifiable.” Another French right-winger, Éric Zemmour, who also scoffed at the odds of a Russian invasion, and made no secret of his admiration for Mr. Putin, has undergone a similar awakening.