On Tuesday, Vladimir Putin admitted what had been known to Western intelligence services (and, surely, Russian ones, too) for some time: a bomb, likely planted by terrorists with connections to ISIS, brought down a passenger plane flying from Sharm el-Sheikh, Egypt, to St. Petersburg, Russia, on October 31st, killing all two hundred and twenty-four people on board. At a meeting of security officials at the Kremlin—the sort of event that is more a show for the Russian state media than a substantive policy session—Alexander Bortnikov, the head of the Federal Security Service (F.S.B.), told Putin that an improvised explosive device equivalent to a kilogram of T.N.T. was to blame. Putin, who appeared genuinely moved by the attack and its implications, called for a moment of silence, and then said of those responsible, “We will find them anywhere on the planet and punish them.” Russian military operations, he added, will prove that “vengeance is inevitable.”

In the days after the tragedy, the Kremlin downplayed talk of terrorism. The minister of transport said that claims by an ISIS affiliate in Egypt to have bombed the plane “can’t be considered accurate.” Earlier this month, when British officials said they had evidence that an explosive had been planted on board and cancelled direct flights from Sharm el-Sheikh, Konstantin Kosachyov, a Russian parliamentarian, claimed that Britain’s real intention was to put “psychological pressure” on Russia. “There are plenty in the world who would prefer to knowingly and without the necessary grounds write off this catastrophe as jihadists’ response to Russia,” he said. That argument showed some strain two weeks ago, when Russia, too, decided to suspend all flights from Egypt. Still, Putin’s spokesman insisted that the cancellation of Egypt flights did “not mean that a terrorist attack is the main suspected cause of the catastrophe.”

Why the lag in announcing what seemed increasingly certain? Most likely, Putin and his advisers simply didn’t know how to react—and when Putin doesn't like his options he tends to stall for time. (That is one advantage of an authoritarian system: there is no legislature or opposition party, and little pressure from the media, to force a decision.)

Had the Kremlin acknowledged that the plane was blown up by ISIS, as an affiliate of the group claimed in the days after the disaster, it would have demonstrated exactly the sort of cause-and-effect it has been trying to avoid in its Syria war. State television has serialized the Russian bombing campaign in the high American style, with splashy news footage provided by military satellites and cameras mounted on warheads, speaking to Russians’ buried yearnings for geopolitical greatness: public support for Russia’s intervention grew from thirty-nine percent in September to fifty-five percent in October, according to the Levada Center, an independent polling agency.

Yet Putin and his advisers seem to have considered this mood potentially fragile, the euphoria of a media-friendly bombing campaign that appeared to cost little and risk nothing. The grim news of two hundred and twenty-four dead countrymen could make a distant war seem awfully close—not only leading to questions about the virtue of Russian policy but also requiring Putin do something in response.

Then came Paris, where terrorists with connections to ISIS killed a hundred and twenty-nine people last Friday. The scenes of carnage changed thinking in world capitals, not least Moscow. Gleb Pavlovsky, a former Putin adviser turned critic, told the Ekho Moskvy radio station that the Kremlin has known for some time the true cause of the crash but chose to stay silent. A terrorist act “would certainly have been perceived by public opinion as a punishment for the invasion of Syria. And that would be a painful thing to deal with,” he said. But then, he went on, “The terrible events in Paris solved this problem.” After Paris, the notion of an ISIS bomb on a Russian plane no longer served as an unwelcome reminder of a faraway war’s potential costs at home. Rather, it proved that Russia and the West faced a single enemy, and that there could be no doubt that the world’s powers would need to unite against an evil so horrible.

Putin’s argument to the rest of the world is similar to the one he is making to his own people. In his telling, Russia tried to cooperate with the United States and others in fighting ISIS—an offer the West rejected. (He neglects to mention that Russia has prioritized targets other than ISIS in Syria, choosing instead to hit other groups that threaten the Assad regime.) “It seems to me that that now, finally, the realization is coming to everyone that only all of us together can fight effectively,” Putin said at the G-20 Summit, in Antalya. He is a adherent of Yalta-era geopolitics, in which states achieve victory together and then sit down to carve up the world. And he wants today’s global powers to form an alliance similar to the one that defeated Nazism.

The Paris attacks, and the prospect that this was not ISIS’s last attack in Europe, seem to be nudging Western powers in this direction. Putin badly wants them to reconsider his pariah status and forgive his meddling in Ukraine. What he may get, if anything, is a tactical alliance to fight ISIS without the lifting of sanctions or the end of Russia’s broader diplomatic isolation. Yet it will be hard for the West to maintain that sort of discipline. If last year’s G-20 summit, held in the wake of Russia’s annexation of Crimea, represented the apex in the public shunning of Putin—so much so that he went home early—then this year’s summit marked his return, for now, to the club of respectable statesmen. President Obama and Putin had a one-on-one meeting that lasted nearly an hour, thus continuing the thaw of the diplomatic deep freeze in which the Obama Administration had once wanted to plunge him. British Prime Minister David Cameron also met with Putin in private. But no public statement was stronger than that of French President François Hollande, who, staying back in Paris, declared that a “single coalition”—including Russia—was needed to fight ISIS.

Questions about the utility of such an alliance come down to what Putin really wants to achieve in Syria, and how much Russian policy overlaps with the interests of the current U.S.-led coalition. Until now, Russia’s bombing campaign has focused not on ISIS targets but on the other rebel groups fighting Bashar al-Assad, serving as the air force to a combined Syrian-Iranian push to recapture territory lost to the rebels. On Wednesday, a military source told reporters at Vedomosti that Russian artillery was providing “covering fire” to Syrian troops in the Homs region. The campaign recalls Russia’s insertion of its soldiers into Ukraine at key moments in the war there: a jolt of military force to give its proxy a battlefield advantage before a likely round of diplomatic negotiations. A Russian draft proposal, obtained by Reuters last week, calls for a constitutional reform lasting up to eighteen months and then early presidential elections.