How might Russia and Syria react to military action?

Though the Syrian air defence system was heavily damaged by Israeli attacks in March, it remains a threat. Its ageing missiles still fly faster than modern jets – a 45-year-old SA 5 missile brought down an Israeli F-16, prompting the Israeli counter-strikes. However, the air defence battery offers little defence to ground-hugging missiles. And more than 100 Israeli strikes had previously been launched without a plane being hit.

The bigger question is whether Russia would activate its more formidable S-400 system, which has been in place in Syria for more than a year. This poses a lethal threat to the world’s most modern jets. During previous attacks, Russia has sometimes briefly turned the system on, but it has not yet used it. A warning by the Russian foreign minister, Sergei Lavrov, to Trump not to attack could indicate a different approach a strike is ordered this time.