As the drum beats of war passed over the ancient city of Damascus yesterday, United Nations weapons inspectors climbed into their distinctive white 4x4 vehicles and headed for the Lebanese border.

Across the city, too, residents braced themselves for the expected onslaught of Tomahawk cruise missiles and sophisticated American and French munitions. Supplies were being bought in and hoarded, families moved into basements and those who listened to the radio heard their leader, president Bashar al-Assad, vowing: "This is a historical confrontation from which we will come out victorious."

Across the world in another time zone, US leaders gave the reasons why their country's armed forces were intent on raining death on the people of Syria. Speaking in the sonorous surroundings of the Treaty Room in the State Department in Washington DC, US Secretary of State John Kerry also summoned history as his witness as he held up a four-page intelligence dossier that he described as a "catalogue of evil". Here, he said, was the evidence that more than 1400 innocent people had perished in a chemical weapons attack in a suburb of Damascus on August 21, including 426 children.

Speaking with controlled passion, Kerry argued that the Syrian regime had committed a crime against humanity that could not go unpunished. "History will judge us extraordinarily harshly if we turn a blind eye," he said. "This is the indiscriminate, inconceivable horror of chemical weapons. This is what Assad did to his own people."

Today Syria is in the cross-hairs after Obama made it clear the US is determined to strike, despite the withdrawal of British support earlier in the week. Kerry had made no mention of his country's oldest ally but instead mentioned the French - reviled 10 years ago as "surrender monkeys" for refusing the back the US-led invasion of Iraq. At the end of a

tense week, here was the evidence the US claim confirms that the Syrian regime had authorised the chemical attack that sparked this latest crisis. Within the next few days - no timetable has been given - Syria will face aerial bombardment to add to the miseries of a civil war that has already lasted two-and-a-half years and claimed about 100,000 casualties.

According to US President Barack Obama, Syria's decision to use chemical weapons represented a "challenge to the world" and to US national security. Obama insisted that he was not talking about a full invasion but would keep operations to a "limited, narrow act". Not only would this punish the Assad regime but it would make clear that as a "leader in the world", the US had a responsibility to hold countries accountable if they violate "international norms".

"This would not be an open ended intervention," he added. "We would not put boots on the ground. Instead our action would be designed to be limited in duration and scope."

As the politicians were speaking in sombre tones in Washington, the guided-missile destroyer USS Stout joined four other warships already stationed in the eastern Mediterranean in preparation for the expected assault. The attacks, using cruise missiles and other "smart" munitions, are expected to target Syrian command and control centres, military headquarters and arms dumps.

When the attacks go ahead, the US will receive limited support from France and Turkey, which have both expressed their disapproval of Assad's regime and the need to take a firm stance against it. Also supporting the attack is the Arab League, and countries including Saudi Arabia and Qatar could provide local back-up. Being Sunni, they have little love for the Shia regime in Syria and have already been supporting the rebel forces.

"It would be a fairly short, sharp action - much like Operation Desert Fox," claims Professor Peter Mansoor of Ohio State University, referring to a similar operation against Iraq in 1998.

This was the preferred strategy of a previous Democrat administration led by former president Bill Clinton, but it failed dismally five years later when US forces pulverised Baghdad with missiles and bombs at the start of the invasion of Iraq in 2003.

Then, as now, military analysts warned that so-called surgical air strikes always needed a further intervention by ground forces and so it proved. The so-called "shock and awe" bombing campaign also caused numerous civilian casualties and senior US commanders have admitted it is impossible to use cruise missiles without the possibility of collateral damage.

The experience of Iraq has cast a shadow over the current crisis but there is one difference: Iraq possessed only rudimentary air defences and land forces equipped with obsolescent Soviet-era weapons. Syria is in a different position. Although its defence forces do not possess missile interceptors like the US Patriot system, they do have sophisticated Russian and Chinese anti-aircraft missile and radar systems that are capable of shooting down hostile planes.

They are also capable of mounting retaliatory attacks against installations used by the US and its allies - the Nato air force base at Incirlik in Turkey is within range of Syrian mobile Scud missile launchers and Syria's ally, Hezbollah, has a variety of mid- range missiles capable of hitting targets in Israel.

Much will depend on the position taken by Iran, Syria's main ally and backer. Last week, the country's Supreme Leader, Ayatollah Ali Khamenei, told members of the Iranian cabinet that US intervention in Syria would be "a disaster for the region … if such an act is done, certainly, the Americans will sustain damage like when they interfered in Iraq and Afghanistan".

Some Iranian fundamentalist groups have been reported as promising to turn the region into a second Vietnam, but the reality is that instigating a wider war would not be in Assad's interests. The most likely reaction to a US missile strike will be an intensification of the fighting against the rebel Free Syrian Army.

Amid continuing uncertainty, this remains the biggest fear. By their nature, limited missile strikes will achieve very little, though they will give some succour to the rebel forces. Perversely, unless they do serious damage to Syria's own armed forces they will also stiffen the morale of forces loyal to Assad. Whatever else US military intervention achieves, it will not stop Assad from fighting the enemy within and could make matters worse. "The Middle East is already divided," says a British diplomatic source. "Further conflict will only make those divisions worse."