Russia-watchers have spent a lot a time wondering: Just what would Vladimir Putin do if he were directly challenged on Syria? Well, now it's happened. Last night, the United States launched a volley of cruise missiles against a Syrian airfield.

This is not one of those situations where Putin had no options. He was warned of the strikes in advance—a good and smart move, given that Russian personnel are stationed at the targeted airbase. With that information in hand, Russia could have tried to intercept America's missiles. It did not. Here's what Putin might be thinking.

Let's start with what his military has been doing on the ground. The Syrian regime operates Russian anti-aircraft systems, but they are older and not able to handle dozens of cruise missiles. The Pentagon says its Tomahawks targeted anti-aircraft weapons at the airbase, meaning those are likely not there anymore. Russia, on the other hand, has moved its best anti-aircraft missile systems, the S-300 and S-400, into Syria. The Russian move seemed to suggest these weapons were there to protect Syrian airbases from U.S. airstrikes.

The thing is, these sophisticated systems have been placed at Russian facilities. According to the Russian ministry, the S-400 and Pantsir systems are stationed at Hmeymim airbase (35°24′42″N 35°56′42″E, near Al Assad Airport) and also at the Russian naval base in Tartus. These are Russian facilities—their prime military assets within Syria.

The Syrian airbase that was targeted, Shayrat, is not too far away. The Russians use that airbase, too, and in late 2015 they improved the runway and moved equipment so that Russian warplanes could stage airstrikes from there. But they didn't need to put air defenses there to protect themselves. It's a safe bet that Russian radar protects the place. It's under the umbrella of the S-300s, which have a range of more than 90 miles, and use a radar system that extends over 185 miles. Tomahawks can be programmed to fly to avoid radar sites, but this pervasive coverage would be hard to skirt.

Besides, Russia knew the swarm was coming. News agencies in Washington are reporting that the U.S. gave at least one hour of warning before the missiles struck. That is plenty of time to fire up those radars, move mobile launchers, and get the best missile crews at their stations.

So Putin could have taken a shot at the Tomahawks. He could have claimed he was saving the lives of Syrian allies who are fighting terrorists. Even if air defenses were overwhelmed by the sheer number of American cruise missiles, a few downed missiles could have served as a rebuttal to this new aggressive posture from D.C., not to mention an information warfare coup. In addition, retaliating against cruise missiles makes the question of escalation less intense, since there will be no dead or wounded American pilots.

But Putin didn't. Maybe this was a tactical choice he made. Maybe the U.S. called his anti-aircraft bluff. Maybe he wanted a message delivered to Assad that his support has limits, and war crimes don't help.

Or perhaps Putin has another game, one of salesmanship.

There is no greater open question in the defense world than just how effective Russian anti-aircraft weapons really are against American technology. Russia generates money and international leverage by selling systems that it claims can thwart American weapons. But the United States' jamming, cyberwarfare, smart missiles, and advanced decoys are designed to defeat these digitally-linked Russian systems. There would be no greater marketing disappointment than shooting at U.S. cruise missiles and missing, which would demonstrate the deterrent Russia is selling may not work as advertised.

Putin could have shot at those Tomahawks, but maybe the risk calculus had more to do with his long-term geopolitical game than a short-term loss of face.

Correction: An earlier version of this article referred to the Tomahawks as ICBMs, which they are not.

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