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 to promote it.

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 bombing a Russian airliner last month. The Kremlin has much to gain: An alliance could mean the end of European sanctions against Russia, which will expire in January unless renewed. The question for Western governments, including a rightly skeptical Obama administration, is whether joining with 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 Assad.

The only productive contribution Putin could make to an anti-Islamic State coalition would be to reverse himself, use Russia’s leverage to obtain the removal of 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.