Again, the costs of these brief gains piled up. As a result of sanctions and falling energy prices, the Russian economy has shrunk to $1.2 trillion from $2 trillion in 2014. And NATO, once an alliance in search of a mission, is now focused again on deterring Russia.

Mr. Putin has also been compelled to abandon the Novorossiya project: His proxies in eastern Ukraine neither enjoy popular support nor run an effective government. And his actions have guaranteed that Ukraine will never join his Eurasian Economic Union or line up with Russia again.

His policy toward another ally, Mr. Assad, has failed, too. Despite blocking Security Council resolutions against Mr. Assad’s government, providing weapons to Damascus and encouraging Syria’s allies to come to the regime’s defense, Mr. Putin’s efforts have done little to strengthen Mr. Assad’s rule. After four years of civil war, Mr. Assad governs less territory and faces more formidable foes. That’s why Mr. Putin had to intervene — to save his autocratic ally from defeat.

In the short term, Russia’s Syrian bombing campaign has energized the Syrian Army and its allies to launch a counteroffensive against opposition rebels — that is, against everyone except the Islamic State. But in the long run, Russian airstrikes alone cannot restore Mr. Assad’s authority over the whole country.

Mr. Putin is adept at short-term tactical responses to setbacks, but less talented at long-term strategy. Even with no response from the West, Mr. Putin’s foreign adventures will finally fail, especially as domestic economic problems continue to fester.

But the United States and its allies should seek to shorten that time by pushing back against Russia on multiple fronts. As Mr. Putin goes all in to prop up his ally in Syria, we should do the same with our partners and allies — not only in Syria, but in Europe and around the world.

In Syria, the United States cannot allow Russia to eliminate all actors except Mr. Assad and the Islamic State. We must provide more arms and support to other rebel groups. We should warn Mr. Putin that further attacks against non-Islamic State rebels will compel us to protect them, either by enforcing a no-fly zone or by arming them with antiaircraft weapons.