What are the potential targets?

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 al-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 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.

Likely 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.
What are US/western capabilities?

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.
What could the UK contribute to any retaliatory effort?

The RAF has a large contingent in the Middle East still engaged in action against Isis remnants and could easily divert planes and drones to Syrian military targets if required. It has Tornados posted in the region that have a reputation for flying low and precision bombing, and Typhoons armed with Paveway lV guided bombs. It also has Reaper drones equipped with Hellfire missiles. All were in action over the last few weeks in either eastern Syria or Iraq against Isis.

However, the main value of the UK to any military action by the US – which is not short of firepower or expertise – would be in allowing the US to say it isn’t acting unilaterally.
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.
What does the Syrian opposition want?

What remains of the opposition is on its knees. It has no defence against air attacks and has lost most of its strongholds. Only Daraa in the south and Idlib in the north remain under its control, and the latter it shares with jihadis who have a very different view of what a postwar Syria should look like. For the past six years opposition leaders have been calling for no-fly zones or airstrikes to weaken the regime.

Russian and Syrian jets have battered anti-Assad forces into submission, eastern Ghouta being the latest example of that. With Russia dominating the skies across at least two-thirds of Syria, a no-fly zone is not an option. The opposition pipe dream remains the ousting of the regime. But with that not a consideration for even Assad’s foes these days, the best the opposition can hope for is a weakened Syrian air force.https://www.theguardian.com/world/2018/apr/10/what-could-the-us-target-in-syria-and-how-might-russia-react