The Battle of Sempach was fought on 9 July 1386, between Leopold III, Duke of Austria and the Old Swiss Confederacy. The battle was a decisive Swiss victory in which Duke Leopold and numerous Austrian nobles died. The victory helped turn the loosely allied Swiss Confederation into a more unified nation and is seen as a turning point in the growth of Switzerland.

The Old Swiss Confederacy or Swiss Confederacy (Modern German: Alte Eidgenossenschaft; historically Eidgenossenschaft, after the Reformation also Corps des Suisses, Confoederatio helvetica "Confederation of the Swiss") was a loose confederation of independent small states (cantons, German Orte or Stände) initially within the Holy Roman Empire. It is the precursor of the modern state of Switzerland.

It formed during the 14th century, from a nucleus in what is now Central Switzerland, expanding to include the cities of Zürich and Berne by the middle of the century. This formed a rare union of rural and urban communes, all of which enjoyed imperial immediacy in the Holy Roman Empire.

This confederation of eight cantons (Acht Orte) was politically and militarily successful for more than a century, culminating in the Burgundy Wars of the 1470s which established it as a power in the complicated political landscape dominated by France and the Habsburgs. Its success resulted in the addition of more confederates, increasing the number of cantons to thirteen (Dreizehn Orte) by 1513. The confederacy pledged neutrality in 1647 (under the threat of the Thirty Years' War), although many Swiss served privately as mercenaries in the Italian Wars and during the Early Modern period.

After the Swabian War of 1499 the confederacy was a de facto independent state throughout the early modern period, although still nominally part of the Holy Roman Empire until 1648 when the Treaty of Westphalia ended the Thirty Years' War. The Swiss Reformation divided the confederates into Reformed and Catholic parties, resulting in internal conflict from the 16th to the 18th centuries; as a result, the federal diet (Tagsatzung) was often paralysed by hostility between the factions. The Swiss Confederacy fell to invasion by the French Revolutionary Army in 1798, after which it became the short-lived Helvetic Republic.