Erik and Hermann had wanted to join infantry units. Their idea of battle was to run at the enemy firing a rifle, and kill or be killed for the Fatherland. But they were not going to be killing anyone. Both had had one year of medical school, and such training was not to be wasted; so they were made medical orderlies.

The fourth day in Belgium, Monday 13 May, was like the first three until the afternoon. Above the roar and snarl of hundreds of tank and truck engines, they began to hear another, louder sound. Aircraft were flying low over their heads and, not too far away, dropping bombs on someone. Erik’s nose twitched with the smell of high explosives.

They stopped for their mid-afternoon break on high ground overlooking a meandering river valley. Major Weiss said the river was the Meuse, and they were west of the city of Sedan. So they had entered France. The planes of the Luftwaffe roared past them, one after another, diving towards the river a couple of miles away, bombing and strafing the scattered villages on the banks where, presumably, there were French defensive positions. Smoke rose from countless fires among the ruined cottages and farm buildings. The barrage was relentless, and Erik almost felt pity for anyone trapped in that inferno.

This was the first action he had seen. Before long he would be in it, and perhaps some young French soldier would look from a safe vantage point and feel sorry for the Germans being maimed and killed. The thought made Erik’s heart thud with excitement like a big drum in his chest.

Looking to the east, where the details of the landscape were obscured by distance, he could nevertheless see aircraft like specks, and columns of smoke rising through the air, and he realized that battle had been joined along several miles of this river.

As he watched, the air bombardment came to an end, the planes turning and heading north, waggling their wings to say ‘Good luck’ as they passed overhead on their way home.

Nearer to where Erik stood, on the flat plain leading to the river, the German tanks were going into action.

They were two miles from the enemy, but already the French artillery was shelling them from the town. Erik was surprised that so many gunners had survived the air bombardment. But fire flashed in the ruins, the boom of cannon was heard across the fields, and fountains of French soil spurted where the shells landed. Erik saw a tank explode after a direct hit, smoke and metal and body parts spewing out of the volcano’s mouth, and he felt sick.

But the French shelling did not stop the advance. The tanks crawled on relentlessly towards the stretch of river to the east of the town, which Weiss said was called Donchery. Behind them followed the infantry, in trucks and on foot.

Hermann said: ‘The air attack wasn’t enough. Where’s our artillery? We need them to take out the big guns in the town, and give our tanks and infantry a chance to cross the river and establish a bridgehead.’