The Treaty of Tolosa was signed in the autumn of 1324 by the rulers of four principalities in the region north of the Pyrenees. It resulted in their unification into a single state, the Kingdom of Aquitaine, which soon became a major power in western Europe.
The five duchies of Aquitaine, Vasconia, Tolosa, Provenca and Barcelona had until recently been subject to the Kingdom of Spain, but from 1308 onwards they were effectively independent after rebelling with foreign aid. In early 1324 Spain, accepting that it would be unable to reconquer the region, recognised their independence in return for trade concessions.
Although now at peace, the region was still threatened by powerful neighbours. In particular, from 1320 to 1323 Lyonesse had tried to subjugate Aquitaine, and was only barely fought off when the other states came to Aquitaine's aid. Four of the dukes therefore agreed to form a union to protect their interests, and to this end they met in Tolosa in July 1324. Barcelona abstained from the negotiations as it wished to maintain its independence, though it did eventually join the union a century later.
The four dukes reached an agreement whereby one of their number would be made king over the others. He would be chosen by a council of twelve ambassadors, three from each state, subject to the Caliph's approval, and his heirs would then reign after him. The other duchies would maintain all their rights, but would be bound to allow free trade within the realm and to support one another diplomatically and militarily against their European neighbours.
Eneko IV, Duke of Vasconia, was eventually chosen to be the first King of Aquitaine, and reigned in relative peace until his death.