The strike violated domestic American law, as there was no congressional authorisation, and it violated international law, specifically Article 2 of the UN Charter, which bars the “threat or use of force against the territorial integrity or political independence of any state”. It was an act of aggression, which is, according to the Nuremberg principles, the “supreme international crime”.
The timing of the strike indicates that it was not motivated by humanitarian concerns, as it is unclear who was responsible for the chemical weapons attack, and a logical step would have been to help coordinate an independent investigation to ascertain the facts and then proceed to bring whoever was responsible to justice using legitimate mechanisms (i.e. via the UN). There is no way that in the short space of time between the chemical attack and the US strike that the Trump administration could have gathered irrefutable evidence as to who the culprit was; certainly nothing that would have justified an illegal military attack. The tragedy in Idlib was clearly a pretext for military intervention, rather than the actual reason.
The US has once again reaffirmed that it has the right to do whatever it wants in the world, without needing to defer to a higher body or seek approval from anyone, which is perfectly consistent with how rogue states operate.
If the principle that it is justified to attack military bases in a sovereign state in order to deter repetition of atrocities was universally applied then military bases in America, Britain, France, Israel, Saudi Arabia and a host of others would all be legitimate targets. Suppose Iran attacked military bases in the UK under the pretext of deterring future aggression of the kind we carried out against Iraq (which resulted in the deaths of hundreds of thousands of people and destroyed the region); how might we react?
If it turns out that Assad was responsible for the chemical attack in Idlib, he certainly wouldn’t be the only one to have used chemical weapons against defenceless civilians; America and Britain used white phosphorous in Iraq, and America notoriously used it in its attack on Fallujah during which 5000 civilians were killed. Israel used white phosphorous shells to bomb hospitals during its 2008/2009 attack on Gaza. During the 1990s, Turkey dropped US-made napalm on Kurdish villages as part of a campaign of ethnic cleansing that was strongly supported by America and Britain. These are just a few examples.
The consequences of the attack are likely to be extremely negative; risking military confrontation with Russia is reckless in the extreme. The threat of nuclear war is increasing, and one provocation on either side could easily set off an irreversible chain of events. The attack has done nothing to curb the Assad regime’s military capabilities, but has allegedly succeeded in killing nine civilians. It could even strengthen the regime, which now has added rhetorical justification for carrying out its crimes by being able to repeat the line that it is under attack from the West and is acting defensively.
More commentators and political officials from both parties in the US will now be calling on Trump to escalate and carry out further strikes once it becomes obvious that the limited attack he has already authorised has had no observable consequences on the ground, and he will probably oblige so as not to appear weak. This could end in full-scale invasion, which would be catastrophic (see: Iraq, Libya).
If it turns out that the regime was responsible for the chemical attack, one step would be for the UN to oversee the destruction of all of the illegal substances and to move in independent monitors to help stem the atrocities. Whatever influence Trump has with Putin he should use to pressure Russia and Syria to get round the negotiating table (maybe in return for relief from sanctions or promise of investment), and America should stop supplying weapons to the rebel forces, whose own actions are often awful, and end up eliciting a more brutal response from the regime. Aid-drops and an increased intake of refugees would be positive steps in the short-term.