What is the purpose of USA warning Russia about missile strikes in Syria?

Question

According to this article, the US warned the Russians ahead of Syria missile strikes. It does not specify when exactly this occurred (a few minutes means nothing, 1-2 hours means that some troops/planes can be moved from the target area).

This article tells us about the Russian Growler anti-missile defense system that theoretically could be used against the tomahawks:

Russians have one of their self-proclaimed state of the art Growler anti-missile defense systems on Latakia Airbase on the Syrian coast. The Growler has reportedly been able to intercept targets at a range of 250 miles and at heights of up to 90,000 feet.

Question: what is the reason to make such a warning? From a military perspective, it does not make any sense. So, I suspect a political reason.

Answer 1

Friend of my enemy is not always my enemy

It's possible that Russia had people on that site, as they have been supporting Assad's regime airforce and air defence systems. Having Russian casualties from a surprise strike is not currently beneficial for USA. Giving an advance warning means that any such people (if they are there) will be evacuated or, if not, allows USA to disclaim responsibility for any Russian casualties.

It appears that having a surprise was not considered essential for this strike.

It's reasonable to assume that the actual destruction of airplanes and airbase facilities is just a secondary goal. The purpose of this strike is political/strategic - a demonstration, a response to the chemical attacks. If some people and material are removed because of the warning, it doesn't harm the primary goal.

Answer 2

Russia was warned because there is a Russian military presence at the base.

Airbus Defence & Space satellite imagery shows that there were four [Russian] Ka-52 Alligator and three Mi-28N Night Hunter helicopters deployed to Al-Shayrat Air Base, 30 km southeast of Homs city, on 31 March. Al-Shayrat has previously been used as a forward base for Russian Mi-24 and Mi-35 helicopters, four of which could be seen at the base on 31 March. (Source: Jane's Defence Weekly)

[T]he Americans told Moscow in advance about the strike, to avoid casualties among Russian military personnel stationed on the airbase (source: BBC)

Sources have told the Guardian that US intelligence officials believe Russian personnel were at al-Shayrat airbase when sarin was loaded on to a Syrian jet. They have not established whether the Russians knew it was happening. (source: The Guardian)

Killing Russian personnel could lead to a huge escalation.

Answer 3

We can not say for sure the purpose unless some U.S. official holds a talk about it. However, there are several considerations here:

1. At the moment, the official U.S. position is that the Assad's regime is responsible for use of chemical weapons. Unless it is proven that Russians committed the attack, they are not a target for military response.

   Tillerson said Russians were not targeted by the strikes. — AP

2. Back in 2015, the U.S. and Russia have agreed a memorandum on preventing air incidents in Syria. They have established a hotline with the purpose of avoiding air collisions between the Russians and the U.S.-led Coalition:

   …the hotline that was established after Russia joined Syria's civil war in 2015 to help Syrian President Bashar Assad's government against opposition groups. The hotline's primary intent is to ensure Russian planes conducting combat missions in Syria's skies don't stumble into an accident or confrontation with aircraft flown by the U.S.-led coalition fighting an Islamic State insurgency in the north of the country. — ABC News

3. The U.S. missile strike was not justified by UNSC; a day before, Russia has vetoed the Resolution. So, even though the memorandum was about preventing mid-air collisions (not missile strikes), it seems to be an important factor for the US to demonstrably follow all necessary precautions to avoid human casualties and further escalation of the conflict.

4. 1-2 hours means that some troops/planes can be moved from the target area

   Officially, there are no Russian air forces in Shayrat air base. If they were, they would also became suspects of committing the chemical attack.
   If, after the warning, activity were noticed, this would be a solid proof that the Russians were more than just "supporting" the chemical attack.

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Also,

Russian Growler anti-missile defense system that theoretically could be used against the tomahawks

The S-400 is not designed to intercept cruise missiles. It simply does not work against the targets flying at heights of below ~60 meters. However, the Shayrat air base was also defended by Pantser-S1 short-range air defense system, at least two of which have been reportedly destroyed (source, YouTube video, in Russian, at timestamp 03:31).

Answer 4

Don't forget that you have (multiple) political pressures and military requirements which don't always match very well. Bombing the airfield and not killing Russian troops could well come from these different positions, resulting in a compromise.

Compatible with this is the concept that the whole attack was a show of strength rather than a debilitating strike anyway. In that case the US's cause is helped, not harmed, by getting across the message that even when you're expecting us to hit your airfield, there's little you can do except run away -- just think what we could do without warning or to put it another way Nice country you got there. Be a shame if anything happened to it.

Answer 5

The Russians were warned because the US government had no wish to create Russian casualties and thereby cause an even deeper political crisis than the one which would inevitably follow from attacking Russia's ally.

Answer 6

This is because of the memorandum of understanding between Russia and the US about informing each other. As a consequence of the strike, Russia suspended the memorandum, so any further strikes will not require informing Russians.

Answer 7

I appreciate the detailed answers but the commonsense since the Cold War needs to be said clearly:

Because that would be a conventional military conflict with Russia, which is feared to escalate into a limited nuclear conflict followed by a total nuclear conflict.

It is reported that as of 2016, Russia can destroy Texas or equivalent size area with one missile. That is roughly 1/1000th of total world land mass with one missile. I pick this missile because I recall hearing about it in 2016 because it made some sensation, probably because American capabilities do not need to make the rounds on American media as much. That is why all-out nuclear conflict is feared.

Also, a limited nuclear war is clearly a nonsensical idea. The totally obvious fact is that if one nuclear warhead is going to go off it is imperative to decimate the opponent's nuclear capability as quickly as possible. It is expected that both sides would likely succeed but each side guards their secrets carefully. Note each side even hides nuclear capability in submarines.

The world has generally regarded it as really, really important for two fully nuclear capable nations to not enter any military conflict. Note that as of August 2016 Donald Trump either disagrees with all of this answer, or says irrelevant things constantly, but he seems to have avoided striking Russians for whatever reason.