The Principality of Achaea was the crusader state in the Peloponnese, the most successful of the Latin conquests in Greece, and the one that lost Mystras to Byzantium as a prince's ransom.
The Principality of Achaea was established in the Peloponnese — the Morea, to its Frankish rulers — by William of Champlitte and Geoffrey of Villehardouin after 1204, and it was the most durable and best-organised of the crusader states in Greece.
It was conquered quickly and with relatively little resistance, partly because the Byzantine provincial administration had already collapsed and local landowners preferred a working government to none.
It ran on French feudal law, produced a substantial literature, built serious castles, and was the wealthiest Latin state in the east after the kingdom of Cyprus. And in 1259 its prince rode to Pelagonia and lost it its dominance in an afternoon.
The conquest of the Morea took about three years and was carried out by a few hundred knights, which says more about the state of Byzantine Greece in 1205 than about Frankish military prowess.
The settlement was systematic. The principality was divided into twelve baronies, each owing knight service, and governed under the Assizes of Romania — a body of feudal law that is one of the fullest statements of crusader legal practice to survive.
Greek landowners were largely left in place and Greek Orthodox worship continued, which is why the state functioned. The Chronicle of the Morea, written in the fourteenth century in Greek, French, Italian and Aragonese versions, is the principality's own account of itself and one of the most interesting sources for how a mixed Frankish-Greek society actually worked.
William of Champlitte led the conquest and left; Geoffrey I of Villehardouin (1209–1229) secured the state and built Chlemoutsi, using — the Chronicle says — money confiscated from the church, for which he was excommunicated and evidently did not much care.
Geoffrey II (1229–1246) was the principality at its height: rich enough to lend money to the Latin emperor in Constantinople and to maintain a court that western visitors described as the most chivalrous in the east.
William II (1246–1278) completed the conquest of the Morea by taking Monemvasia, and then lost the principality's position at Pelagonia — captured in the battle and released only in 1262 in exchange for the fortresses that became Byzantine Mystras.
William II joined the coalition of Epirus and Sicily against Nicaea in 1259 because a Byzantine restoration was an obvious threat to a Frankish principality in Greece. He was right, and he lost.
Taken prisoner at Pelagonia, he spent three years in captivity and bought his release by ceding Mystras, Monemvasia and Maina, in the south-east of the peninsula.
Those fortresses became the base of a Byzantine province that grew into the Despotate of the Morea and spent the next two centuries taking the peninsula back, castle by castle. Mystras became a Byzantine capital and one of the great centres of late Byzantine art and scholarship.
The principality declined through the fourteenth century under absentee Angevin and Navarrese rulers, and the Byzantines took most of what remained before the Ottomans took everything in 1460.
What survives is the castles — Chlemoutsi, Mistra's lower town, Karytaina, Acrocorinth — and the Chronicle, which is the closest thing the Latin east produced to a national literature.
It also demonstrates something the crusader states elsewhere rarely managed: a Latin ruling class and a Greek population coexisting for two centuries in a working polity. It did not save it, but it is why it lasted as long as it did.