The Prussians retreated 18 miles north to Wavre whilst Wellington's Anglo-Allied army withdrew 15 miles north to a site he had noted the previous year as favourable for a battle: the north ridge of a shallow valley on the Brussels road, just south of the small town of Waterloo. On 17 June there was torrential rain, which severely hampered movement and had a considerable effect the next day, 18 June, when the Battle of Waterloo was fought. This was the first time Wellington had encountered Napoleon; he commanded an Anglo-Dutch-German army that consisted of approximately 73,000 troops, 26,000 of whom were British. Approximately 30 percent of that 26,000 were Irish. The Battle of Waterloo commenced with a diversionary attack on Hougoumont by a division of French soldiers. After a barrage of 80 cannons, the first French infantry attack was launched by Comte D'Erlon's I Corps. D'Erlon's troops advanced through the Allied centre, resulting in Allied troops in front of the ridge retreating in disorder through the main position. D'Erlon's corps stormed the most fortified Allied position, La Haye Sainte, but failed to take it. An Allied division under Thomas Picton met the remainder of D'Erlon's corps head to head, engaging them in an infantry duel in which Picton fell. During this struggle Lord Uxbridge launched two of his cavalry brigades at the enemy, catching the French infantry off guard, driving them to the bottom of the slope, and capturing two French Imperial Eagles. The charge, however, over-reached itself, and the British cavalry, crushed by fresh French horsemen hurled at them by Napoleon, were driven back, suffering tremendous losses. A little before 16:00, Marshal Ney noted an apparent exodus from Wellington's centre. He mistook the movement of casualties to the rear for the beginnings of a retreat, and sought to exploit it. Ney at this time had few infantry reserves left, as most of the infantry had been committed either to the futile Hougoumont attack or to the defence of the French right. Ney therefore tried to break Wellington's centre with a cavalry charge alone. At about 16:30, the first Prussian corps arrived. Commanded by Freiherr von Bülow, IV Corps arrived as the French cavalry attack was in full spate. Bülow sent the 15th Brigade to link up with Wellington's left flank in the Frichermont–La Haie area while the brigade's horse artillery battery and additional brigade artillery deployed to its left in support.