It has taken almost two years, but a pattern to Donald Trump's behaviour is finally emerging. He has a powerful ability to shock and to dominate headlines. He's quite easily provoked and likes to hit out. But his attention span is short and he is easily distracted. So even if he does launch an air strike on Syria, as he promised a few days ago, there is no real prospect of his starting a longer campaign. He can be expected to deplore the barbarity of last week's chemical weapons attack, fire a few missiles and then walk away.

This, anyway, is likely to have been Bashar al-Assad's calculation last week in the attack on Douma in Eastern Ghouta, which looks to be the latest in his chemical weapons campaign. At the time, the atrocity was greeted with astonishment as well as horror. Trump had only recently decided to withdraw American troops from Syria, so why would Assad do anything to pull the US back in? Why not just keep hostilities to a minimum while America retreats?

This image released early Sunday, April 8, 2018 by the Syrian Civil Defense White Helmets, shows a child receiving oxygen through respirators following an alleged poison gas attack in the rebel-held town of Douma. Credit:AP

Any use of chlorine gas would be certain to provoke Trump because he defines himself against Barack Obama. When a chemical bomb killed 1,400 in Damascus four years ago, Obama failed to respond - so Trump's instant reaction was to promise swift and firm revenge. It is a fairly standard pattern of behaviour: if he feels challenged, he will respond. After punishing a Syrian chemical weapon attack last year, he had no choice but do the same now - with an alliance of the willing, from the French to the Saudis. Assad's tactics could be seen as lunacy.

Until you look at what Trump means by "hitting hard". George W Bush once put it well: the military is not there to underline a verbal point. No president, he said, should be prepared to "fire a $2 million missile at a $10 empty tent and hit a camel in the butt" - if you act, it needs to be "decisive".