Duke Leopold III, after he unsuccessfully tried to establish a cheap peace, decided to assemble his forces in order to save possessions and honor of his house. With the help from Tyrol and Italy his army was considerably stronger than that of his uncle Leopold I of Austria at Morgarten. His strength was estimated to be between 3000 and 4000 men.
But the Confederation's army was also probably twice as strong as at the Battle of Morgarten and numbered somewhere between 6000 and 8000 men, since it was made not only of men from the four forest cantons, but also other Swiss cantons (Lucerne, Zürich).
Leopold wisely decided not to attack principal places and turned to the small town of Sempach, some north of Lucerne. He assembled his army at Sursee, about down from Sempach, then surrounded Sempach on the evening of July 8. His men taunted those behind the walls of the village, and a knight waved a noose at them and promised them he would use it on their leaders. Another mockingly pointed to the soldiers setting fire to the ripe fields of grain, and asked them to send a breakfast to the reapers. From behind the walls, there was a shouted retort: "Lucerne and the allies will bring them breakfast!" Leopold did not take the direct route to Lucerne, but rather turned east.
The Confederation army had presumably assembled at the bridge over the Reuss River at Gisikon. It marched from there, hoping to catch Leopold still at Sempach where he could be pressed against the lake. Around noon, the two armies made contact near Sempach, close to the village of Hildisrieden. The battlefield has been definitely proved to be by the old battle chapel.
As the knights of Leopold's army approached, they dismounted and sought to storm the high ground. Their marksmen then took the Swiss under heavy fire. Leopold reasonably believed that the Swiss army lay before him, and engaged in battle before his rear units moved up from the approaching column. But it was only the confederate's advance guard that they were fighting.
The main body of the Confederation army finally completed its deployment from the marching column, formed up, and attacked the knights from the flank aggressively. The attack was so powerful that the knights fighting on foot were immediately overrun and the soldiers, who were holding knights' horses, took flight and the mounted Habsburg army was carried away by the fugitives.
Duke Leopold and with him a large number of nobles and knights were slain.
Arnold von Winkelried is a legendary figure. According to 16th century accounts, he opened a breach in the Habsburg lines by throwing himself into their pikes, taking them down with his body so that the confederates could attack through the opening.
The Lucerne chronicle shows a hill scattered with the fashionable pointy footwear of the Austrian knights, which they had to take off as they dismounted, and which the Confederates found orphaned after the Austrian defeat.