After the 2017 Khan Sheikhoun attack, Donald Trump ordered a strike on the airfield that launched the Syrian jets involved. The 59 cruise missiles launched from US warships in the Mediterranean damaged runways and hangars at Shayrat but they were quickly repaired. That attack was largely symbolic.
If Trump orders a second strike, it will probably be more comprehensive. The US president will be keen to assert himself as a strongman if he feels his own red line has been crossed (comparisons to Barack Obama are too much for him). France too has previously said it would be prepared to act if the use of chemical weapons is proven.
Probable targets a second time round would be other Syrian airbases, and perhaps what remains of the air force itself. Whittled down and battered, the Syrian jet fleet has been heavily propped up by Russian fighters. Syria’s air defence system has been heavily damaged by Israeli attacks.
The US maintains a naval battle group in the eastern Mediterranean, well stocked with over-the-horizon missiles. It has a large number of jet fighters in Qatar and on carriers in the Gulf, which are deployed to bomb Isis. The politics of flying over Iraq or Saudi Arabia to bomb the Syrian regime may prove tricky. Missiles are a more likely option. They’re harder to shoot down and it matters little if they are. Striking from the west poses fewer problems all around. French jets could hit Syrian targets after taking off from French airfields. If Britain joins the fray, it has a base on nearby Cyprus, a short hop from Syria.