After every defeat, a beaten army retreats along its line of supply. This was military gospel in 1815. The Prussians had been driven from the field at Ligny; therefore the Prussians would fall back east, toward Liege and the Rhine, away from Wellington's Anglo-allied force stranded at Quatre Bras. Napoleon believed this. So did Marshal Grouchy, whom he detached on the morning of 17 June with 33,000 men and orders to pursue the Prussians east. But the Prussians did not retreat east. Blucher's chief of staff, Gneisenau - who privately distrusted the British and would have preferred to abandon them - was overruled by his bedridden commander. Blucher, still battered from being trampled by his own dragoons the previous night, made one of the consequential decisions of the nineteenth century. The army would retreat north to the small town of Wavre, parallel to the Anglo-allies, not perpendicular to them. From Wavre, three of his four corps could march west to reinforce Wellington. The fourth would screen the move. The Prussians slipped away through the night in good order, taking their wounded, leaving behind only their ammunition wagons - which they would catch up with at Wavre by the following evening.

Marshal Emmanuel de Grouchy was not, by general consensus, a great commander. He was a competent cavalry officer who had been promoted to the marshalate by Napoleon only weeks before. On the morning of 17 June he was given an enormous detachment - III and IV Infantry Corps, II Cavalry Corps, roughly a third of the entire Army of the North - and orders that were urgent in tone but vague in substance. Pursue the Prussians. Maintain contact. Do not let them join Wellington. Grouchy began slowly. The roads were poor; the rain was beginning; nobody knew quite where the Prussians had actually gone. By noon his cavalry scouts had reported Prussian columns at Gembloux. He moved to Gembloux. By evening of 17 June he was still trying to determine whether the Prussian main body had retreated toward Namur or toward Wavre. He sent a despatch to Napoleon at ten in the evening conceding both possibilities. At two in the morning he sent another saying he intended to advance on either Corbais or Wavre. By the time these despatches reached Napoleon, the question was already irrelevant - because Grouchy was no longer behind the Prussians. He was beside them, on the wrong side of the river Dyle, and unable to either catch them or rejoin the emperor in time for the battle the next day.

At eleven thirty on the morning of 18 June 1815, Marshal Grouchy sat down to breakfast at the house of a notary named Hollaert in the village of Sart-lez-Walhain. He had ridden ahead of his main column. The rain had finally stopped. From somewhere to the northwest came a low, continuous rumble - artillery, a great deal of it. His chief of staff Colonel Loriere heard it from the garden. He fetched the marshal. Grouchy stepped outside with his subordinates - the gruff Vandamme, the cavalryman Exelmans, and most consequentially General Etienne Maurice Gerard, commander of IV Corps. Hollaert was asked to identify the direction of the sound. He pointed north-northwest and said: it comes from the Forest of Soignies, from the direction of Mont-Saint-Jean. Gerard knew exactly what that meant. The emperor was in a major battle, only about twenty-three kilometers away across country. March to the guns, Gerard urged. I will go with my corps. We can be there by afternoon. Grouchy refused. His orders were to pursue the Prussians. He believed - wrongly - that the Prussian main body was still in front of him. He believed - correctly - that the country between him and the battle was bad, the river Dyle was swollen, and his force could not cross it intact. So he continued north, toward Wavre, toward the Prussian rearguard, toward irrelevance. Gerard would never forgive him.

While Grouchy was deciding not to march to the guns, the Prussians were marching to them. Three of Blucher's four corps were already pushing west from Wavre toward the battle. The route they had chosen led through the narrow valley of the Lasne, a small stream that the previous night's rain had turned into a mud-clogged trench between steep banks. Cannon had to be manhandled across by ropes. Horses lost their footing. Infantry slid down one bank and clawed up the other. The Prussian advance, which Blucher had promised Wellington at dawn would arrive by noon, slowed to a crawl. At one point Bulow, commanding the leading IV Corps, found his guns simply stopped - and the seventy-two-year-old Blucher rode up the line of march on horseback, shouting encouragement, telling his men he had given his word as a soldier and they would not make him a liar. By four in the afternoon the head of Bulow's corps emerged from the Wood of Paris, on the eastern edge of the Waterloo battlefield. From there the Prussians could see what Napoleon had not yet seen - that the right flank of the French army was entirely undefended. They began to deploy.

Meanwhile, at Wavre itself, Grouchy was fighting an entirely separate battle. He arrived in front of the town in mid-afternoon on 18 June and attacked the Prussian rearguard - one corps under Thielmann, about 17,000 men, holding the bridges over the Dyle. The fighting was savage and unnecessary. Thielmann was meant to delay; he delayed. Grouchy pressed; he pressed. The battle continued into the night, paused with darkness, resumed at dawn on 19 June. By late morning the next day, Grouchy had pushed Thielmann out of the town and across the river. It was a tactical French victory. It also no longer mattered. The previous evening, while Grouchy's guns were still firing into Wavre, the courier had ridden up from Waterloo with news of Napoleon's defeat. The marshal received it at dawn. He had to extract his intact corps from a battle he had just won, in country crawling with hostile Prussian patrols, and march it back to Paris before the Coalition cavalry caught him. He managed this with considerable skill. It is the one part of his role in the campaign that historians admire.

The forty-eight hours between Ligny and Waterloo decided the Napoleonic Wars. Napoleon's strategic error - assuming the Prussians would behave conventionally - was compounded by a tactical error he made on the morning of 18 June, when he failed to send patrols east to discover Bulow's approach across the Lasne. By the time Prussian columns appeared on the French right flank at half past four that afternoon, it was too late to detach a serious force to meet them. Grouchy's decision at Sart-lez-Walhain has been argued over for two hundred years. Could he have reached Waterloo? Probably not in time to fight, but possibly in time to deter the Prussians from committing fully. Would it have changed the battle's outcome? Wellington thought so. So did Napoleon, who in exile on St. Helena would return again and again to the same bitter sentence: he should have marched to the guns. The notary's house at Sart-lez-Walhain still stands. The fields between Wavre and Plancenoit are now farmland, crossed by motorways and dotted with small monuments. On a summer afternoon, when the wind is from the southeast, it is just about possible to imagine the sound that Hollaert pointed toward, the sound that decided everything.