So Erik had at first rejoiced when the frost had set in at the beginning of November. The freeze seemed a blessing, making the roads hard again and allowing the ambulance to move at normal speed. But Erik shivered in his summer coat and cotton underwear – winter uniforms had not yet arrived from Germany. Nor had the low-temperature lubricants needed to keep the engine of his ambulance operating – and the engines of all the army’s trucks, tanks and artillery. While on the road, Erik got up every two hours in the night to start his engine and run it for five minutes, the only way to keep the oil from congealing and the coolant from freezing solid. Even then he cautiously lit a fire under the vehicle every morning an hour before moving off.

Hundreds of vehicles broke down and were abandoned. The planes of the Luftwaffe, left outside all night on makeshift airfields, froze solid and refused to start, and air cover for the troops simply disappeared.

Despite all that, the Russians were retreating. They fought hard, but they were always pushed back. Erik’s unit stopped continually to clear away Russian bodies, and the frozen dead stacked by the roadside made a grisly embankment. Relentlessly, remorselessly, the German army was closing in on Moscow.

Soon, Erik felt sure, he would see Panzers majestically rolling across Red Square, while swastika banners fluttered jubilantly from the towers of the Kremlin.

Meanwhile, the temperature was minus ten degrees Centigrade, and falling.

Erik’s field hospital unit was in a small town beside a frozen canal, surrounded by spruce forest. Erik did not know the name of the place. The Russians often destroyed everything as they retreated, but this town had survived more or less intact. It had a modern hospital, which the Germans had taken over. Dr Weiss had briskly instructed the local doctors to send their patients home, regardless of condition.

Now Erik studied a frostbite patient, a boy of about eighteen. The skin of his face was a waxy yellow, and frozen hard to the touch. When Erik and Hermann cut away the flimsy summer uniform, they saw that his arms and legs were covered with purple blisters. His torn and broken boots had been stuffed with newspaper in a pathetic attempt to keep out the cold. When Erik took them off he smelled the characteristic rotting stink of gangrene.

Nevertheless, he thought they might yet save the boy from amputation.

They knew what to do. They were treating more men for frostbite than for combat wounds.

He filled a bathtub, then he and Hermann Braun lowered the patient into the warm water.

Erik studied the body as it thawed. He saw the black colour of gangrene on one foot and the toes of the other.

When the water began to cool they took him out, patted him dry, put him in a bed and covered him with blankets. Then they surrounded him with hot stones wrapped in towels.