The Battle of Wavre was the final major military action of the Hundred Days campaign and the Napoleonic Wars. It was fought on 18–19 June 1815 between the Prussian rearguard, consisting of the Prussian III Corps under the command of General Johann von Thielmann (whose chief-of-staff was Carl von Clausewitz) and three corps of the French army under the command of Marshal Grouchy. A blocking action, this battle kept 33,000 French soldiers from reaching the Battle of Waterloo and so helped in the defeat of Napoleon at Waterloo. Background Following defeat at the Battle of Ligny two days earlier, the Prussian army retreated north in good order and formed up at Wavre. Wellington's Anglo-allied army won at Quatre Bras, enabling them to move northwards, to a defensive position at Waterloo. Napoleon moved the bulk of his army off in pursuit of Wellington, and sent Grouchy in pursuit of the retreating Prussians with the right wing (aile droite) of the Army of the North (L'Armée du Nord), a force consisting of 33,000 men and 80 guns. The French units in the order of battle were: III Corps (General Dominique Vandamme) 17,099 infantry – 38 guns IV Corps (General Étienne Maurice Gérard) 15,013 infantry – 38 guns II Cavalry Corps (General Remy Exelmans) 3,392 infantry – 12 guns IV (Hussars) Cavalry Division (General Pierre Soult) 1,485 infantry – 8 guns detached from the I Cavalry Corps 5,000 cavalry from the Reserve Army Grouchy was slow in taking up the pursuit after Ligny, which allowed Prince Blücher to fall back largely unmolested to Wavre, regroup his army, and then execute a flank march with three of his four corps to join up with Wellington's Anglo-allied army at Waterloo. The remaining corps, the III Prussian Corps (Thielmann's) of 17,000 men and 48 guns, was to follow the other three corps leaving a small rearguard in Wavre, unless the French appeared in force in which case he was to oppose any French attempt to close on the main body of the Prussian army. Prelude French positions Marshal Grouchy was in Gembloux with III Corps commanded by General Vandamme and IV Corps commanded by General Gerard. The 4th Cavalry Division, commanded by Pajol, and the 21st infantry division, under Teste (from Lobau's corps), formed the remainder of his force. Reconnaissance by Pajol's horsemen during the 17th of June found that the Prussians had left Namur. French