A sudden sheet of flame tore up into the sky a short distance behind the head of the enemy column and Napoleon saw scores of men hurled aside by the blast. The red flame of the explosion faded and a rolling mushroom cloud billowed above the Austrian lines. He saw a crater in the road and scores of blackened bodies and body parts lay scattered around it.

‘Jesus,’ Napoleon muttered in horror. ‘What was that?’

‘Must have been an ammunition wagon,’ Berthier replied. ‘Lucky shot from our side must have set it off.’

The explosion caused a brief lull in the fighting.The Austrians had turned towards the sound of the blast, dazed and frightened. At that moment a trumpet call sounded from Napoleon’s left and he turned and saw that the small body of cavalry covering the left flank was moving, picking up speed as it surged forward down the side of the enemy column, and then wheeled inwards towards the Austrians, still shaken by the blast.

‘The young fool!’ Berthier said through clenched teeth. ‘He’ll get himself killed.’

Napoleon strained his eyes and realised that the cavalry formation belonged to Kellermann, the son of the hero of Valmy, and one of Murat’s most promising officers. Napoleon shook his head. ‘No, he’s done the right thing. It’s perfectly timed. Look!’

Kellermann’s troopers launched themselves into a charge, trumpets blaring and colours rippling in the wind as they spurred their mounts forward, extending their heavy swords until the glinting blades pointed directly at the terrified Austrians in front of them. A few had the presence of mind to turn and fire their muskets at the charging horsemen; then they were engulfed by the French cavalry and the Austrian column shattered. Men threw down their heavy muskets and ran, fleeing back down the road towards Marengo, and away from the advancing lines of the men of Victor’s and Lannes’s divisions. As far as Napoleon could see the battlefield was covered with white-coated figures streaming away from their French pursuers. Large bodies of men, still in column, laid down their arms and surrendered and their colours were snatched from their hands by jubilant Frenchmen.

As dusk gathered over the battlefield Napoleon made his way forward with Berthier. There was a thick belt of bodies where Marmont’s guns had torn into the leading battalions of the enemy column and then two lines of corpses where Desaix’s men had exchanged volleys with the enemy before they had finally broken. From the earliest reports to have reached headquarters it seemed that over five thousand of the enemy had been killed and an even larger number taken prisoner, along with forty guns, fifteen colours and General Zach, the second in command of the Austrian army. Nightfall, and the presence of strong detachments of Austrian cavalry, had ended the French pursuit and across the plain the exhausted men were re-forming their units and marching back to camp.