Sparring over Ukraine created a gulf; countering Islamist terrorism is bridging it. At the G20 summit last weekend, Vladimir Putin had cordial meetings with Barack Obama and David Cameron, and spoke of Russia and the West joining forces to fight Islamic State. That was a sharp contrast to last year’s meeting, when the Russian president was snubbed and left early. Yesterday the Kremlin acknowledged, as Western governments long claimed, that terrorists indeed brought down a Russian airliner over Egypt late last month. Promising retribution, Mr Putin also told his military commanders that the French, who earlier this year cancelled a deal to sell two Mistral helicopter-carriers to Russia’s navy, should be treated as “allies”. François Hollande, the French president, will visit Moscow on November 26th. There’s nothing like a common enemy to turn rivals into partners. How far will it go? Ukraine and other frontline states are watching nervously.