Mr Trump is another reason to be cautious. His presidency has not been burdened with ethical pretence, and it is hard to take seriously his sudden respect for human rights in international affairs. The president called out the “horrible attack” by “Animal Assad” but seemed distracted by an FBI raid on his lawyer’s office. No wonder he is confused: it was only last week that Mr Trump ordered his generals to arrange a rapid exit of the 2,000 US troops who have been advising anti-regime rebels. Britain would gain little from hitching our foreign policy responses in this most complex of conflicts to such a shameless and mercurial figure. The fact is that airstrikes or a barrage of missiles might have a demonstration effect, but they would not change the military balance on the ground. Nor would a single attack alter the outcome of the war.