Standing beside Secretary of State Rex Tillerson at the State Department on Wednesday, Foreign Minister Sergey V. Lavrov of Russia gleefully mocked his American hosts. A reporter had just shouted a question: “Does the Comey firing cast a shadow over your talks?” Mr. Lavrov, feigning shock, replied, “Was he fired?” And then, “You’re kidding! You’re kidding.” As he turned to walk out of the ornate reception area, the minister fought a losing battle with the wolfish grin overtaking his face.

Five thousand miles away in Sochi, Russia, President Vladimir V. Putin, in full ice hockey gear, cheerfully answered a similar query from an American reporter. “Your question looks very funny for me,” he said. “Don’t be angry with me. We had nothing to do with that.”

Russia’s leaders have earned their moment of mirth. A campaign to undermine the legitimacy of the West’s democratic institutions and leaders has succeeded in the United States itself beyond anything they could have imagined. In nine months, the virus unleashed by Moscow has managed to sow doubt about the American electoral system; help defeat Hillary Clinton, the candidate Mr. Putin despised; bring down President Trump’s first national security adviser; prompt multiple counterintelligence investigations; and now, cause Mr. Trump to fire the man in charge of investigating him, further shredding his already self-tattered credibility.

This success is all the sweeter because Mr. Trump, wittingly or not, has aided and abetted Russia’s efforts every step along the way. His repeated claims this fall that the election was rigged echoed and amplified propaganda spouted by the Russian networks RT and Sputnik. So, too, his widely debunked postelection allegations of widespread voter fraud.