President Putin’s aggressive election meddling, like his broader foreign policy, has at least two motivating factors. The first is his conviction that the surest path to restoring Russia as a great power comes at the expense of an American-led order. He wants Russia unconstrained by Western values and institutions, free to pursue a sphere of influence.

The second motivating factor is closely connected to the first. The legitimacy of Mr. Putin’s system of repressive domestic control depends on the existence of external threats. Surfing on high oil prices, he used to be able to bolster his social contract with the Russian people through rising standards of living. That was clear in the boomtown Moscow I knew as the American ambassador a decade ago, full of the promise of a rising middle class and the consumption of an elite convinced that anything worth doing was worth overdoing. But Mr. Putin has lost that card in a world of lower energy prices and Western sanctions, and with a one-dimensional economy in which real reform is trumped by the imperative of political control and the corruption that lubricates it.

The ultimate realist, Mr. Putin understands Russia’s relative weakness, but regularly demonstrates that declining powers can be at least as disruptive as rising powers. He sees a target-rich environment all around him.

If he can’t easily build Russia up, he can take the United States down a few pegs, with his characteristic tactical agility and willingness to play rough and take risks. If he can’t have a deferential government in Kiev, he can grab Crimea and try to engineer the next best thing, a dysfunctional Ukraine. If he can’t abide the risk of regime upheaval in Syria, he can flex Russia’s military muscle, emasculate the West, and preserve Bashar al-Assad atop the rubble of Aleppo. If he can’t directly intimidate the European Union, he can accelerate its unraveling by supporting anti-Union nationalists and exploiting the wave of migration spawned in part by his own brutality. Wherever he can, he exposes the seeming hypocrisy and fecklessness of Western democracies, blurring the line between fact and fiction.

So what to do? Russia is still too big, proud and influential to ignore and still the only nuclear power comparable to the United States. It remains a major player on problems from the Arctic to Iran and North Korea. We need to focus on the critical before we test the desirable. The first step is to sustain, and if necessary amplify, the actions taken by the Obama administration in response to Russian hacking. Russia challenged the integrity of our democratic system, and Europe’s 2017 electoral landscape is the next battlefield.