Today, Mr. Putin presents Russia’s actions as responsive, not aggressive. Every time Russia attacks a former Soviet republic, the confrontation is portrayed as a proxy war started by America against Russia. When Russia attacked Georgia in 2008, the United States was in the midst of a presidential election that the incumbent Republican Party would soon lose, so the war was followed the next year not by tough sanctions against Russia but with a “reset” initiated by the new Democratic president, Barack Obama, and his secretary of state, Hillary Clinton.

That, too, proved a misstep. The idea was that two new presidents — Mr. Obama and Dmitri A. Medvedev, who had recently taken Mr. Putin’s place in the Kremlin — could put the past behind them. But Mr. Medvedev, who oversaw the war in Georgia, was only a place-holder appointed by Mr. Putin to circumvent a constitutional term limit. Policy makers in Washington and in Berlin knew this but decided to build up Mr. Medvedev, hoping to split Russia’s elites. Instead, Russia got away with the Georgia war cost-free, which ultimately contributed to Russian confidence that its later incursions into Ukraine would succeed.

Mr. Medvedev’s presidency ended with mass demonstrations in Moscow and other cities in the winter of 2011-12, with tens of thousands protesting Mr. Putin’s return to the presidency and demanding modernization of the state. At the time, Mr. Putin accused Mrs. Clinton of taking “active measures” to spur protesters on. “She set the tone for some actors in our country and gave them a signal,” he said. “They heard the signal and with the support of the U.S. State Department began active work.”

Now Mr. Putin, who is known to bear grudges, appears to be disrupting Mrs. Clinton’s own presidential campaign with “active measures.” That the disclosures of the Democratic National Committee emails could benefit only Donald Trump is probably an added bonus. Mr. Trump’s main appeal within the Kremlin is not that he admires Mr. Putin, but that he has little interest in Russia’s sphere of influence. And Mr. Putin has long dreamed of a new Yalta-style agreement to let Russia and America divide Europe again.

To be sure, Russia’s ability to influence American elections is limited. Mr. Putin does not control the American media, and Russia lacks the financial and military resources that the Soviet Union had. Still, the effort points to a danger. An angry and declining Russia is far more perilous than an ascending economic power like China. Sanctions won’t change Mr. Putin’s behavior: He rates the security of his regime far above the economic good of the country.

Mr. Putin has reason to fear in one respect. His system does face an existential threat from the Western model of governance. Just as the economic inadequacies of Soviet Communism were exposed by comparison with the wealth produced by Western capitalism, Mr. Putin’s authoritarianism cannot match the appeal of an economy based on the rule of law, openness and competition. The best way for the West to resist Russia, now as then, is to uphold its own values.

“Russia, as opposed to the Western world in general, is still by far the weaker party and may well contain deficiencies which will eventually weaken its own total potential.” So wrote the master diplomat George F. Kennan in 1947. We know now that Kennan was right. The bad news is that it took 44 years for his prophecy to come true.