All the signals from Washington suggest that military action against Syria is a strong possibility. Contingency plans are being drawn up, potential target lists are being reviewed and various military assets are being moved into position.

The US Navy is re-positioning several vessels, including four cruise missile-carrying destroyers in the eastern Mediterranean and probably a missile-firing submarine.

If more firepower is needed, two US aircraft carriers could launch air strikes, and land bases in Turkey and Cyprus might also be used. French air power could also play a part.

But what kind of military action is being proposed ? What risks are involved ? What is the rationale behind such action? And, perhaps most importantly, how might Western military action contribute to a resolution of the Syrian crisis, if at all?

Forces which could be used against Syria:

•Five US destroyers - USS Gravely, USS Ramage, USS Barry, USS Mahan and USS Stout - are in the eastern Mediterranean, equipped with cruise missiles. The missiles can also be fired from submarines, but the US Navy does not reveal their locations.

•Airbases at Incirlik and Izmir in Turkey, and in Jordan, could be used to carry out strikes

•One aircraft carrier - USS Nimitz with three destroyers and a cruiser have moved into the southern Red Sea. A second aircraft carrier USS Harry S Truman is in the wider region.

•French aircraft carrier Charles de Gaulle is currently in Toulon in the western Mediterranean

•French Rafale and Mirage aircraft can also operate from Al-Dhahra airbase in the UAE

'Mission-creep'

The military options facing US political leaders are varied, ranging from a short, sharp punitive strike against targets in Syria (perhaps the most likely) to - at the other end of the spectrum - a full-scale intervention, including ground troops, to try to end the country's civil war.

This is not in any sense on the table at the moment, though it is the "shadow" lurking off-stage.

Models for possible intervention Iraq 1991: US-led global military coalition, anchored in international law; explicit mandate from UN Security Council to evict Iraqi forces from Kuwait

US-led global military coalition, anchored in international law; explicit mandate from UN Security Council to evict Iraqi forces from Kuwait Balkans 1990s: US arms supplied to anti-Serb resistance in Croatia and Bosnia in defiance of UN-mandated embargo; later US-led air campaign against Serb paramilitaries. In 1999, US jets provided bulk of 38,000 Nato sorties against Serbia to prevent massacres in Kosovo - legally controversial with UN Security Council resolutions linked to "enforcement measures"

US arms supplied to anti-Serb resistance in Croatia and Bosnia in defiance of UN-mandated embargo; later US-led air campaign against Serb paramilitaries. In 1999, US jets provided bulk of 38,000 Nato sorties against Serbia to prevent massacres in Kosovo - legally controversial with UN Security Council resolutions linked to "enforcement measures" Somalia 1992-93: UN Security Council authorised creation of international force with aim of facilitating humanitarian supplies as Somali state failed. Gradual US military involvement without clear objective culminated in Black Hawk Down disaster in 1993. US troops pulled out

UN Security Council authorised creation of international force with aim of facilitating humanitarian supplies as Somali state failed. Gradual US military involvement without clear objective culminated in Black Hawk Down disaster in 1993. US troops pulled out Libya 2011: France and UK sought UN Security Council authorisation for humanitarian operation in Benghazi in 2011. Russia and China abstained but did not veto resolution. Air offensive continued until fall of Gaddafi Models for possible intervention

Those who are sceptical of military entanglement fear that any action could escalate. Western forces might get drawn into a more protracted struggle, "mission-creep", risking an open-ended military commitment that many fear might be as dangerous as another Iraq or Afghanistan.

So what are the military options?

The US Chairman of the Joint Chiefs of Staff, Gen Martin Dempsey, gave his most detailed view in a letter to Senator Carl Levin in mid-July.

This is the most authoritative assessment of the military options as seen by the Pentagon that is available in an unclassified form.

Let's look at each of these, though not necessarily in the order that Gen Dempsey discussed them. Bear in mind that these are not mutually exclusive; combinations of different options could well be employed.

1. Limited stand-off strikes

Some might call these punitive strikes.

The aim would be to get Syrian President Bashar al-Assad's attention and to persuade him not to resort to chemical weapons in the future. Targets could include military sites linked closely to the regime - the headquarters or barracks of elite units, for example.

Missile production facilities could be hit. Caution would have to be exercised if striking chemical weapons production facilities since leakage of toxic chemicals could lead to significant local damage.

Air defence sites and command centres might also be hit as a warning of Western capabilities should there need to be recourse to military action again in the future.

The attraction of this option is that it could be mounted quickly and with limited risk to the Western forces involved. The weapons of choice would be Tomahawk land-attack cruise missiles launched from US Navy warships and submarines.

This could be scaled up using air-launched weapons, but again these would be "stand-off" in nature (i.e. launched from well outside Syrian airspace). French aircraft could strike targets in Syria while operating from their home bases, as they did during the crisis in Libya and Mali.

2. Stepped up support to the Syrian opposition

Gen Dempsey actually put this as his first option. This would involve non-lethal force to step up the training and advice to elements of the opposition. This would be an extension of some of the work that has already been under way.

However, this approach has already foundered upon the growing divisions within the opposition and the rising fears in the West that some of the most effective fighters on the ground come from groups with links to al-Qaeda-type organisations.

3. Establish a 'no-fly' zone

The aim here would be to prevent the Syrian government from using its air power to strike rebels on the ground and to re-supply isolated bases around the country. This would probably require Syria's air defence system to be dismantled, and forces would have to be available to shoot down Syrian military aircraft that took to the skies.

Such a no-fly zone has been discussed for well over a year and generally rejected. Much has been made of Syria's air defence system, which before the civil war was extensive and well-integrated. It is made up of large numbers of Soviet-era weapons with a significant sprinkling of much more modern Russian systems.

However, the effectiveness of this system as a whole is in doubt. Territorial losses to the rebels mean that some key sites have been lost to the government, and the Israeli air force has demonstrated that it can hit targets inside Syria with impunity (though some of the strikes may well have been made using stand-off weapons).

What is clear is that establishing a no-fly zone would involve much greater initial risk to US and allied aircraft and it would require the assembling of a significant force - which would have to be maintained over time - not just combat aircraft, but tankers, airborne command (Awacs) aircraft and so on.

4. Establish buffer zones

The idea here would be to establish havens inside Syria - probably close to its borders with Turkey and Jordan - from which rebel forces could operate and within which refugees could be supplied. Again this is an option that has been previously discussed and rejected.

Such safe havens might require the establishment of a limited air exclusion zone, and there would be serious questions as to how they might be defended on the ground. If, for example, the Syrian government fired into the zones, what then?

Another idea that has sometimes been mentioned is a no-drive zone, effectively limiting the use of President Assad's ground forces. But here, too, air power would be needed and this option begins to look very much like embarking upon a full-scale war in Syria.

5. Control Syria's chemical weapons arsenal

This was Gen Dempsey's fourth point with a focus on preventing the use or proliferation of chemical weapons. This could be done by destroying portions of Syria's stockpiles; hindering its movement or by seizing key installations. This would require a massive US involvement, including troops on the ground, for an indefinite duration.

What comes through clearly in Gen Dempsey's letter (and indeed in a subsequent text that he recently sent to another US congressman in mid-August) is his extraordinary reluctance to embark upon any military action at all.

Image caption Free Syrian Army fighters inspect munitions and a tank that belonged to forces loyal to the Syrian government

That was, of course, before the suspected use of chemical weapons in Syria, and President Barack Obama being forced by circumstances to confront the "red line" of his own choosing.

The most likely scenario, if force is to be used, is number (1) above - a short, sharp punitive strike to send a message to the Syrian regime. But any decision to act raises all sorts of questions: