The US missile strike on a Syrian airbase came days after the chemical attack that killed scores in Idlib but years after the evidence began piling up of brutality, torture, the deliberate targeting of civilians, medical facilities and aid and the repeated use of chlorine by forces fighting to defend Bashar al-Assad’s regime. Both those who have long called for his forcible removal and those who believe any military intervention to be wrong and dangerous may well ask what has changed. The uncertainty is supercharged by the fact that the attack was ordered by Donald Trump, a volatile narcissist without a coherent worldview, moral compass or significant attention span, who loudly urged Barack Obama not to take action after Mr Assad’s forces used sarin to kill more than 1,000 people at Ghouta in 2013.

These questions matter. But the international community has long agreed chemical weapons are particularly abhorrent, and sarin (invisible and extremely potent) especially so. It has been used only a handful of times, notably by Saddam Hussein’s forces at Halabja in 1988. Mr Assad himself signed up to the chemical weapons convention after Ghouta, forced by the US threat of military strikes. His commitment to destroying his stockpile was soon proved a lie: some arms remained, and he used chlorine repeatedly, breaching the convention. Emboldened by the indifference to his campaign of terror, his troops now appear to have used sarin again.

Deploying military force in response – particularly unilaterally, without UN approval – will always, rightly, be controversial. The risks are still more serious in a long and complex war involving Russia and Iran. Nerves and rumours across the region are jangling. So far, and the administration’s unpredictability dictates caution, this has been presented as a discrete, largely symbolic action, sending a message on chemical weapons specifically. The US has targeted the airbase it says was used to launch the sarin attack. It warned the Russians in advance, with the aim of avoiding a confrontation. Many – including the UK, Turkey, Saudi Arabia and Israel – have supported it.

Inevitably, the strike is being read in the light of Steve Bannon’s departure from the US national security council and the rise of national security adviser HR McMaster and defence secretary James Mattis (and, more broadly, Mr Bannon’s struggles with Jared Kushner for influence). It has thus been seen as a victory for “globalists” who want to reassert that America will uphold its values internationally and for experienced “grown-ups”, who have coordinated a clear but calibrated response. That seems premature.

Mr Trump’s decision was apparently driven in part by his emotional reaction to photos of Tuesday’s attack. Compassion is welcome in a president; selective, impulsive responses less so. But look at the context too. A president facing historically low poll ratings will probably see a boost; military action is usually popular, in the short term at least. US allies were already concerned he might be looking for a fight, albeit probably with Kim Jong-un. A man who likes to boast of decisiveness, but with little to show for 11 weeks in office, ordered action that could not be blocked by Congress or the courts. A politician facing an unprecedented scandal over his campaign’s dealings with Russia and Moscow’s interference in the election has put distance between himself and the Kremlin. A poll bump could encourage him to take rash action elsewhere – perhaps Pyongyang. And, of course, it opens the door to further action in Syria, even if that is not envisaged now.

The broader ramifications remain to be seen. Russian anger may affect the campaign against Islamic State. Moscow has no great love for Mr Assad, but does not see an obvious alternative; replacing him may become less likely, since it will not want to appear to have been strong-armed. Rex Tillerson’s meetings in Russia next week may offer indications of the Kremlin’s response to a more assertive administration. Mr Trump’s willingness to press the button may make North Korea more cautious and Chinese president Xi Jinping, who was dining at Mar-a-Lago as missiles soared, may consider more pressure on Pyongyang. But he will not like the attack’s timing, nor want to look weak by offering significant concessions in the short term.

On its own, this action is unlikely to stop the sustained and systematic crimes against humanity. There were reports of further bombing at the Idlib site today. It may deter the regime from using sarin again and from indulging in the most public excesses of viciousness. It may discourage others from employing such weapons. Those are important goals – however partial and imperfect. But Mr Trump’s inconsistency and amorality is epitomised by his attempts to bar Syrian refugees. Though an ignored red line has been partially redrawn, we are no closer to a solution to Syria’s tragedy; nor to the reassertion of a US vision of a rules-based order, however flawed. A world defined by Mr Trump’s impulses cannot be a safe one.