LONDON Any strike by the United States and its allies on Syria will probably aim to teach President Bashar al-Assad - and Iran - a lesson on the risks of defying the West, but not try to turn the tide of the civil war.
U.S. and European officials say a short, sharp attack - perhaps entirely with cruise missiles - is the preferred response to what they believe is Assad's responsibility for a chemical weapons attack on rebel-held areas last week.
If such a strike goes ahead, President Barack Obama's administration will have to select its targets with extreme care as it tries to deter not only Assad but also Syria's ally Iran over its nuclear programme.
"The administration has to decide what its objective is - punishment to show that there is a price and to re-establish a deterrent, or to change the balance of power in Syria," said Dennis Ross, a top White House adviser on the Middle East until late 2011. "I suspect it will be geared towards the former."
NATO air strikes in 2011 helped to change the course of the Libyan civil war, allowing rebels to overthrow Muammar Gaddafi, but Obama is unlikely to opt for something similar in Syria.
U.S. officials said the Pentagon has submitted a range of possible attack plans for Syria to the White House, and analysts believe the scope would be modest.
"I think it will happen but it will be minimal, just enough to show the world that we did something," said Hayat Alvi, lecturer in Middle Eastern studies at the U.S. Naval War College. "The broader goal is not to get the U.S. involved too deeply - and especially not to allow any boots on the ground."
Defense Secretary Chuck Hagel said the U.S. military is ready to act immediately should Obama order action.
The United States and its allies were strengthening their forces in the region even before hundreds of people were killed in rebel-held suburbs of Damascus last Wednesday. Syria has blamed the rebels but Washington, London and Paris say they have little doubt it was a chemical strike by Assad's forces.
Without some action soon, officials worry that Assad will feel he can resort to chemical weapons again with impunity - a year after Obama declared their use a "red line" that, if crossed, would require strong action.
Some also fear inaction in Syria could cast doubt over other U.S. "red lines", encouraging Iran to pursue a nuclear programme which Tehran says is peaceful but the United States and its allies including Israel believe aims to produce weapons.
Any failure to strike Syria could also prompt Israel to take matters into its own hands by attacking Iranian nuclear facilities, causing yet more upheaval in an already highly unstable region.
Choosing targets is fraught with danger. The most likely, officials say, would be Assad's command and control facilities, air defences and any part of his chemical arsenal they believe can be attacked safely.
What must be avoided is any action that, while designed to punish the use of chemical weapons, perversely ends up releasing dangerous materials into the environment. Likewise if any technicians from Russia, a major arms supplier to Assad, were killed, this would inflame already troubled Western relations with Moscow.
Defence sources say U.S. commanders want overwhelming force and a robust regional coalition available to deter any Syrian retaliation. Turkish Foreign Minister Ahmet Davutoglu said on Tuesday that last week's attack was a "crime against humanity" which should not go unanswered.
Current and former Western officials, including those directly involved in policy, say Syria's sophisticated air defences and worries about the risk of casualties among allied aircrew meant the cruise missile strike was now most likely.
A "stand-off" attack could be launched from U.S. warships or aircraft firing missiles without entering Syrian airspace. The United States says it has raised the number of destroyers that carry cruise missiles in the Mediterranean to four.
Its most powerful ship in the region - the aircraft carrier USS Harry S Truman - left the Mediterranean on Aug, 18, passing south through the Suez Canal into the Red Sea, although it could still be within striking range.
Defence sources say Britain has kept an attack submarine in the Mediterranean for several months, allowing it to join any U.S.-led missile barrage, just as it did with Libya.
The French carrier Charles de Gaulle has just been declared operational after a refit. Currently in the Mediterranean port of Toulon, it could be off Syria within three days.
LEAST WORST OPTION
Most officials who talked to Reuters said the possibility of allied and civilian casualties was a top consideration.
"It's about the least worst option," said a European defence source on condition of anonymity. "No one wants the risk of pilots being captured or killed."
Manned aircraft could still be ultimately used - Israeli jets have already raided Syrian targets on several occasions, proving it is possible.
U.S. F-16 jets have remained in Jordan after an exercise earlier this year. The U.S. air force could also reinforce its Turkish airbase at Incirlik while B2 long-range bombers could fly from the continental United States, unseen by Syrian radar.
France retains Rafale and Mirage jets at a base in the United Arab Emirates. British, French and other aircraft could operate from bases in Cyprus, although the island's foreign minister said on Tuesday he did not expect Britain's Akrotiri base to play a major role in any strike.
Gulf and other regional allies might provide useful intelligence, Western officials said, although their direct involvement in initial strikes was seen unlikely. The main focus would be protecting them from any retaliation by Damascus.
Syria's conventional forces still pack considerable punch, experts say, including anti-ship missiles that could hit vessels nearby in the Mediterranean and conventional rockets that could hit neighbouring countries including Israel.
Last year, Assad promised not to use chemical weapons within Syria's borders - but explicitly threatened foreign countries if they attempted to impose outside "regime change". Western officials believe Syria retains considerable stocks including VX gas, regarded as much more lethal than the sarin suspected to have been used in last week's attack near Damascus.
Such worries were a major factor in Turkey and Jordan requesting U.S. and NATO Patriot missile batteries now based along the border to shoot down enemy missiles. (Additional reporting by Matt Spetalnick and David Alexander in Washington and John Irish in Paris; editing by David Stamp)