"All glory be to God on high, And to the earth be peace," they sang, but the musicians wore assault rifles and helicopters clattered unseen overhead.
It was the most extraordinary carol service. Several hundred marines and soldiers of the 40 Commando Battlegroup, the Prime Minister singing along with them in the chilly night air of Camp Price, Helmand Province.
David Cameron announced on Wednesday the British contingent in Afghanistan would be reduced by 4000 over the next year.
That’s 4000 or so families who will not now spend six months worrying about their loved ones coming back injured or worse.
Some 500 are already on their way back and should be back for Christmas. Before the carol service, Mr Cameron saw their vehicles being prepared for transport home at Camp Bastion and he insisted that the Afghans are ready to take the strain as British forces draw down.
Afghan capability is certainly growing; the Afghan National Police dealt with 26 IEDs in one day earlier this month and the Afghan National Army are now getting artillery training.
But will they hang together as NATO leaves? Can they cope when things go wrong? Will they even see Helmand as a place worth controlling?
The Government hopes they will, NATO hopes they will, but nobody knows for sure.