The Federal Charter or Letter of Alliance (in German Bundesbrief) documents the Eternal Alliance or League Of The Three Forest Cantons (in German Ewiger Bund der Drei Waldstätten), the union of three cantons in what is now central Switzerland, formed in early August, 1291.

This inaugural confederation grew through a long series of accessions to modern Switzerland. The Alliance was concluded between the people of the alpine areas of Uri, Schwyz and Unterwalden ( homines vallis Uranie universitasque vallis de Switz ac communitas hominum Intramontanorum Vallis Inferioris). The participants are referred to as conspirati and (synonymously) coniurati, traditionally translated in German as "Eidgenossen".

The league was set up as a league for defense purposes against any attacker, probably prompted by the death of Rudolf I of Habsburg on 15 July 1291. Before his death, Habsburg attempted to reinforce his claim over Schwyz and Unterwalden which meant a succession of military interventions.

The authenticity of the letter is disputed. Most historians agree that it is almost certainly a forgery of the 14th century. In 1991, the parchment has been radiocarbon dated to between 1252 and 1312 (with a certainty of 85%). If the document is indeed a forgery, it would therefore be a product of the decades following the described event, and is certainly unrelated to the emergence of the modern federal state in 1848, as had sometimes been suggested before the carbon dating.

The letter should rather be seen in the context of chapter 15 of the Golden Bull of 1356, where Charles IV outlawed any conjurationes, confederationes, and conspirationes, meaning in particular the city alliances (Städtebünde), but also other communal leagues that had sprung up through the communal movement in medieval Europe.

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• Original text and translations

History of Switzerland | 1291 | Manuscripts | Federalism

Bundesbrief | Bundesbrief von 1291 | Pacte fédéral | Foedus Pactum Helveticum | Bondsbrief van 1291 | Patg Federal | Večný spolok