Russia and the United States are once again squaring off over Syria following the use of nerve gas in Khan Sheikhoun on April 4 and a U.S. cruise missile strike on Shayrat Air Base on April 6. This has led to renewed speculation over whether the potent S-400 surface-to-air missiles Moscow has deployed in Syria could effectively counter future U.S. air operations. The S-400, after all, has the range to shoot down aircraft operating over most of Syria, as well as parts of neighboring Turkey and Israel.

Could the S-400 potentially shoot down U.S. war planes? Yes! Could the SAM systems withstand a concerted U.S. campaign to suppress them? No! The more pressing question is, under what circumstances would Russian troops actually fire missiles at American aircraft?

Moscow would have a lot to lose from intentionally starting a war. The anti-aircraft missiles, however, might be authorized if a local commander believes Russian forces in Syria are under attack.

Both sides have been careful to avoid direct hostile fire against each other. For example, Washington notified Moscow in advance of the strike on the Shayrat Air Base. Russia, for its part, did not attempt to shoot down the Tomahawk missiles as they approached the airbase, most likely at Putin’s command.

But accidents in target identification are extremely common in war. For example, U.S. warplanes accidentally hit Syrian troops in September 2016, killing sixty-two. U.S. Special Forces narrowly avoided being hit by a Russian air strike on March 1, while three Turkish soldiers were not so fortunate on February 3. And U.S. warplanes accidentally killed eighteen members of an allied Syrian force this week.

The tension following the Shayrat attack caused Russia to publicly close the communication channels used to deconflict Syrian airspace. Fortunately, this appears to have been a mere rhetorical ploy. Moscow announced that communications would soon reopen.

Nonetheless, it is possible to conceive of a variety of worrying scenarios in which U.S. missiles or aircraft are perceived as targeting Russian forces, causing the SAMs to go into action. Responsible national leadership might be able to step in and prevent an escalation to war.

At any rate, the S-400s in Syria serve primarily as a deterrent. Like brandishing a pistol to scare off a teeming mob, the S-400s in Syria are meant to intimidate potential adversaries because of their potential to inflict harm—but in the event they actually open fire, are unlikely to evade retribution.

Russia has spent decades developing multilayered air-defense systems, each tier of weapons capable of handling different targets at varying altitudes and ranges. The bottom layer is made up of point-defense vehicles capable of engaging low-flying aircraft at short ranges as they approach frontline troops or key installations, a role performed in Syria by Russia’s Pantsir S-1 (NATO codename: SA-22 Greyhound) air-defense vehicles. One level up, there are medium-range missiles that can strike targets at higher altitudes a few-dozen miles away, such as the Buk-M (SA-17 Grizzly) missile involved in the Malaysian Flight MH 17 incident.

The S-300 and -400 family of missile systems, starting with the S-300P first deployed in 1978, constitute the long-range end of the spectrum. The have the range to be considered anti-access/area-denial (A2/AD) weapons capable of denying the airspace of an entire region, and can also, in theory, take out incoming cruise missiles and shorter-range ballistic missiles. A specialized missile-defense variant, the S-300V4, was deployed to the Russian naval base at Tartus in October 2016.

In the last decade, Russia has also begun fielding the evolved S-400 Triumf missile system (SA-21 Growler), which can launch 40N6 missiles with a maximum range of up to 250 miles, at up to six times the speed of sound. This means the S-400 battery stationed at the air base in Latakia can threaten the airspace over nearly all of western and central Syria, and even directly over the airbase in Incirlik, Turkey, where a large chunk of the U.S. Air Force’s combat power in the region is based.

However, the 40N6 missiles are designed to engage less agile adversaries than supersonic fighters—think AWACS, refueling tankers and other airliner-size targets. Fighters would instead be targeted with slower (Mach 2.9), but more maneuverable 9M96E2 missiles, which have a maximum range of seventy-five miles. There is also an intermediate 48N6DM missile with a range of 160 miles.

An S-400 battery can deploy in five to ten minutes and supposedly engage up to thirty-six targets at a time. However, the S-400s are not ideal for hitting low-flying aircraft using the terrain to mask their approach—that’s a job for nearby Pantsir-S1s.

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