By, Prof Michael Clarke:
As the US and UK governments continue to discuss their potential response to the suspected chemical weapons attack in Douma in Syria, what could military intervention achieve?
The critical military virtue of surprise has long since disappeared for the United States and its allies in the strikes it is planning against Syrian military facilities.
Indeed, Syrian forces have had more than two days to move their aircraft and other military assets into Russian bases at Latakia, Tartus and Khmeimim, where they will be within the protective bubbles of Russia's highly capable S-400 surface-to-air missiles.
The Syrians have emptied their infantry bases and dispersed as much of their armed forces as possible, in anticipation of incoming Western missiles.
The Russians will undoubtedly try to protect their bases, if attacked, so the situation is fraught with superpower brinkmanship and the danger of accidental conflict.
For Western military planners the two greatest questions are what can they achieve militarily in this situation, and what strategic difference can it make?
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With Syrian forces forewarned, dispersed and under Russian protection, Western strikes will have to concentrate on Syria's fixed military facilities - bombing runways, destroying buildings and capital equipment where it remains in place.
Western attacks will probably try to destroy Syria's military command and control system, possibly with bunker-busting bombs and deep penetration warheads. They are likely to try to dismantle the military infrastructure that Syria has effectively rebuilt since 2015.
More ambitiously, and also more risky, the United States might declare a longer-term policy of revisiting these targets to keep them out of use and have Syrian aircraft bottled up inside their Russian bases - in effect trying to operate a quasi "no-fly zone" in Syria, at least for a while.
In April 2017 the US launched 59 cruise missiles at the Shayrat airbase in response to a previous chemical weapons attack
Last year when the US struck President Assad's Shayrat airbase in retaliation for the use of chemical weapons in Khan Sheikhoun, the Syrian air force made sure it was seen to be back in action within a day. The US will be determined that this does not happen again, which is why we can expect this to be a more prolonged air campaign with repeated attacks on key sites.
This analysis piece was commissioned by the BBC.
Professor Michael Clarke is a senior research fellow at the Royal United Services Institute for Defence and Security Studies (Rusi), and associate director of the Strategic Studies Institute.