Achaea was founded in 1205 by William of Champlitte and Geoffrey I of Villehardouin, who undertook to conquer the Peloponnese on behalf of Boniface of Montferrat, King of Thessalonica.

With a force of no more than 100 knights and 500 foot soldiers, they took Achaea and Elis, and after defeating the local Greeks in the Battle of the Olive Grove of Koundouros, they became masters of the Morea.

Achaea was rather small, consisting of the Peloponnese peninsula (then known as the Morea), but it was fairly wealthy, exporting wine, raisins, wax, honey, oil and silk.

It was bordered on the north by Epirus and the Duchy of Athens and surrounded by Venetian-held territories in the Aegean Sea, including the forts of Modon and Coron on the Peloponnese.

The resulting register was presented at a parliament held at the princely residence at Andravida, and divided the country into twelve baronies, mostly centred around a newly constructed castle—a testament to the fact that the Franks were a military elite amidst a potentially hostile Greek population.

[6] Aside from Kalamata (and later Arcadia), which became the Villehardouins' personal fief, the Prince's own domain encompassed the region of Elis, where the capital Andravida, the port of Glarentza (Clarence) and the fortress of Chlemoutsi (Clermont) were situated, Corinthia, with the Acrocorinth as the chief site, as well as most of Messenia and Laconia around the fertile valley of Eurotas.

[11][12] With the Byzantine recovery of the region around Mystras after 1261, however, the rapid extinction of the original families and the expansion of Achaean influence across Frankish Greece, the initial organization of the Principate changed.

When John III of Nicaea besieged Constantinople in 1236, Geoffrey II came to the aid of the Latin Empire with 100 knights, 800 archers and 6 vessels.

William was a poet and troubadour, and his court had its own mint at Glarentza, and a flourishing literary culture, using a distinct form of spoken French.

After Michael recaptured Constantinople in 1261, William was released in 1262 in return for Mistra and much of Laconia, which became a Byzantine province (the nucleus of the future Despotate of the Morea), as well as an oath of allegiance to the Emperor.

A proposal to marry William's elder daughter Isabella to Andronikos, eldest son of Michael VIII, was strongly opposed by the Achaean nobility, who had no desire to come under Byzantine rule.

In return for the military aid and funds they so greatly needed, Charles obtained the suzerainty over Achaea from Baldwin, and the Principality itself from William.

However, his son-in-law Philip had died in 1277 without an heir, and a reversionary clause in the Treaty of Viterbo provided that the Principality would go to Charles of Anjou, rather than Isabelle, should this occur.

[21] A renewed commitment by Charles to retake the Latin Empire (Treaty of Orvieto, 1281) was forestalled by the War of the Sicilian Vespers, and this struggle with the Crown of Aragon consumed the remainder of his life.

Not long after his release and coronation in 1289, he granted the Principality to Isabelle of Villehardouin upon her marriage with Florent of Hainaut, in part to redress the grasping application of the Treaty of Viterbo at William's death.

[22] For this period the principality was under a violent succession dispute, which originated from the dispossessed Latin Emperor Baldwin II's gift of the overlordship of Achaea to Charles I of Sicily, in return for support in his attempt to reconquer the throne in Constantinople.

The Angevin kings of Naples subsequently gave Achaea as their fief to a series of their own relatives who fought against Princess Margaret of Villehardouin and her heirs.

The son of Ferdinand and Isabelle, known as James the Unfortunate, was proclaimed prince of the Morea in 1315 under the regency of his father, who conquered the principality between 1315 and 1316 but was defeated and executed by Louis of Burgundy and Matilda in 1316.

In 1373 Philip II transferred his rights to his cousin, overlord and former sister-in-law Queen Joan I of Naples, whose third husband James IV of Majorca, when he died in 1375, left her his own claim to the principality, at which point she became more or less the uncontested Princess of Achaea.