Why did French cavalry charges fail to break the Allied line at Waterloo?

At the Battle of Waterloo, the French cavalry charges failed to break the Allied line primarily because they were launched without adequate infantry or artillery support against well-prepared defensive formations.1

After Marshal Michel Ney mistakenly interpreted the movement of wounded men and empty wagons from the Allied center as a sign of retreat, he ordered massed cavalry attacks against the British lines. In response, the British infantry formed infantry squares—hollow, multi-ranked defensive formations specifically designed to repel cavalry.1

These squares proved highly effective, as the French horsemen were unable to break through the disciplined ranks. Furthermore, the French cavalry were subjected to heavy fire from English gunners as they attempted to navigate the slopes between the strongpoints of Hougoumont and La Haye Sainte. Despite their determination and repeated attempts, the French cavalry could not overcome the combination of the defensive squares and the concentrated fire of the Allied artillery.1