Will the night sky of Damascus soon be illuminated by the explosion of American cruise missiles striking government buildings and military bases as used to happen periodically in Baghdad between 1991 and 2003? Pillars of fire would suddenly spring up on each side of the Tigris river and bursts of tracer rounds from anti-aircraft guns would rise slowly and ineffectively into the sky.

The firing of Tomahawk cruise missiles from four American destroyers in the Mediterranean at targets in Syria are among the actions being telegraphed ahead by the US, Britain and France as the most likely form of retribution for the Syrian army’s alleged chemical attack on civilians in Damascus.

The units and bases from which the US believes rockets carrying poison gas were fired will be probable targets. So too would be Syrian airfields and probably the bases of elite units frequently deployed against the rebels.

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If these attacks do take place, with Britain and France in a supporting role, then President Barack Obama will make them heavy enough to be more than a slap on the wrist but not so devastating that they herald the US becoming a participant in the war. It will not be an easy balancing act: ineffective air strikes that the Syrian government can shrug off would be a demonstration of weakness rather than strength. But strikes by missiles and possibly military aircraft will mean the US is crossing a Rubicon, committing itself more than ever before against President Bashar al-Assad and in favour of the armed opposition. This may mean that if there are missile strikes they will be limited in their timescale but heavier and more destructive than expected.

It is not probable, however, that an air campaign could closely emulate lengthy action in Kosovo in 1999 or in Libya in 2011, both of which have been cited as examples of successful military interventions by Nato. The political situation today is different. The Syrian government is a harder nut to crack than Muammar Gaddafi and his ramshackle state. It has strong regional allies in Iran and Hezbollah who see the struggle for Syria as a battle for their very survival. Syria is also a test case for Russia, which has so far firmly supported Mr Assad and is bidding to regain something of the international influence it had before the fall of the Soviet Union 20 years ago.

Arming the rebels on the ground is the other recipe for punishing the Syrian government. That is already happening with 400 tons of arms, mostly shoulder-fired anti-tank weapons and ammunition, paid for by Saudi Arabia and arriving in northern Syria from Turkey.

Anti-tank weapons from Saudi Arabia are said to have been crucial in the opposition’s capture of Mannagh helicopter base north of Aleppo on 6 August. The most effective force in the battle was the al-Qa’ida-linked Islamic State of Iraq and the Levant (Isil), which began the final assault with a suicide bombing carried out by a Saudi. This success emphasises a further problem faced by increased Western or Saudi arms supplies: to whomever they are initially given, they are likely to end up in the hands of Isil or the jihadist al-Nusra Front.

The Syrian army has a tight grip on most of Damascus and the roads leading north to Homs and west to Tartous on the coast. There are checkpoints every few miles on the main roads which minutely examine documents.

Many of the rebel-held areas such as villages between Homs and Hama are largely empty because they have been heavily bombarded by artillery and from the air. The same is true in many of rebel-held districts in Damascus which have been sealed off, are short of food and have many buildings in ruins. Similarly places like Baba Amr and Qusayr, once rebel strongholds, are now ghost towns while Sunni villages at Houla are cut off.

What stops the Syrian army capturing many rebel areas is not armed opposition but shortage of troops, unwillingness to suffer casualties among trained soldiers and an inability to hold captured areas in the long term. If Syrian generals did use chemical weapons last Wednesday this lack of manpower might explain why they did so.