Russian President Vladimir Putin, right, in Moscow on Tuesday. (Alexey Nikolskiy/Sputnik/Kremlin Pool/European Pressphoto Agency)

THE PARIS attacks created a tactical opportunity for Vladi­mir Putin. For two months the Russian ruler sought to persuade Arab and Western nations to join what he described as an alliance against the Islamic State, even as a Russian offensive in Syria targeted Western-backed Syrian rebel forces. He was spurned, and his military campaign bogged down. Now, in the wake of Paris, French President François Hollande suddenly has become a convert to the grand-alliance idea; he has scheduled visits to Washington and Moscow next week to promote it.

Mr. Putin is doing his best to look like a potential partner. On Tuesday, after weeks of obfuscation, his government suddenly confirmed that the Islamic State was responsible for the bombing of a Russian airliner last month, and Russian forces carried out a rare wave of attacks against the Islamic State capital, Raqqa. The Kremlin has much to gain: An alliance could mean the end of European sanctions against Russia, which will expire in January unless renewed, and the concession of a Russian say over the future of Syria and perhaps also Ukraine, where Russian-backed forces have resumed daily attacks.

The question for Western governments, including a rightly skeptical Obama administration, is whether joining with Mr. Putin would help or hurt the cause of destroying the Islamic State. For now, that’s not a hard call. Russia has little to offer the U.S.-led coalition in military terms, even if it proved willing to focus its attacks on the Islamic State rather than rebels fighting the regime of Bashar al-Assad. At the same time, Mr. Putin’s strategy of bolstering rather than removing the Assad regime is, along with Iran’s similar strategy, the single biggest obstacle to defeating the jihadists.

Russia has sought to demonstrate in Syria that its military forces have been modernized since they struggled to defeat Chechen rebels a decade ago. But military analysts haven’t been impressed with the Russian-led assault on anti-Assad forces in northern Syria. Moscow’s planes have mostly dropped dumb bombs, while Syrian and Iranian troops have lost scores of Russian-supplied tanks and armored vehicles to the rebels’ U.S.-made TOW missiles. Having failed to recapture significant territory, the Russian mission appears doomed to quagmire or even defeat in the absence of a diplomatic bailout.

Mr. Putin duly dispatched his foreign minister to talks in Vienna last weekend on a Syrian political settlement. But Moscow and Tehran continue to push for terms that would leave Mr. Assad in power for 18 months or longer, while — in theory — a new constitution is drafted and elections organized. Even a U.S. proposal that Mr. Assad be excluded from the eventual elections was rejected, according to Iranian officials.

Secretary of State John F. Kerry was rather elegant in explaining the dangers of accepting Russian terms. If the West “cut a deal” such that “Assad can be there for a while longer,” he said, “the war won’t stop.” The Syrian dictator “has become the magnet for the foreign fighters” joining the Islamic State, Mr. Kerry said. His atrocities, from chemical weapons to “barrel bombs,” have convinced the vast majority of Syrian Sunnis that he — and not the terrorists — is their principal enemy.

The only productive contribution Mr. Putin could make to an anti-Islamic State coalition would be to reverse himself, use Russia’s leverage to obtain the removal of Mr. Assad and stop attacks on Western-sponsored forces. Failing that, an alliance with Russia would be a dangerous false step for the United States and France.