In 1259 the army of Nicaea destroyed a coalition of Epirus, Achaea and Sicily at Pelagonia, capturing the prince of Achaea and opening the road to Constantinople.
After the Fourth Crusade took Constantinople in 1204, the Byzantine world broke into pieces: a Latin Empire in the capital, Greek successor states at Nicaea, in Epirus and at Trebizond, and a scatter of crusader principalities in Greece.
By 1259 the Empire of Nicaea under Michael VIII Palaiologos was the strongest of them and was clearly preparing to take Constantinople back. Everyone who preferred that not to happen made an alliance: Michael II of Epirus, William II of Villehardouin, prince of Achaea, and Manfred of Sicily.
They met the Nicaean army under Michael's brother John Palaiologos on the plain of Pelagonia in Macedonia. The coalition collapsed before the battle and was destroyed in it, and two years later Constantinople was Byzantine again.
The Empire of Nicaea had spent fifty years rebuilding: John III Vatatzes had restored its finances and pushed its frontier into Europe, and by the 1250s it held Thessalonica and much of Thrace and Macedonia.
Michael VIII had taken power at Nicaea in 1258 as regent and then co-emperor for the child John IV Laskaris, in a coup that is the beginning of the archive's difficulty with him.
The coalition against him was an alliance of convenience between people with nothing else in common: a Greek despot who claimed the imperial title himself, a French prince of the Morea, and the illegitimate son of Frederick II ruling Sicily. It held together for exactly one campaign.
The armies faced each other on the plain in the summer of 1259. What happened next is disputed in detail and clear in outline: the allies quarrelled, and Michael II of Epirus withdrew his forces in the night — the Byzantine sources say he was outmanoeuvred by Nicaean disinformation, and it is at least clear that he judged the alliance not worth dying for.
That left the Frankish knights of Achaea and the German troops Manfred had sent to fight without their Greek contingent. The Nicaean army, strong in horse archers, engaged on ground of its choosing.
The Frankish cavalry was destroyed and William II of Villehardouin was taken prisoner — found, according to the Chronicle of the Morea, hiding under a haystack and identified by his prominent teeth.
The immediate consequence was strategic. The only coalition capable of stopping a Nicaean move on Constantinople had been destroyed, and Michael VIII was free to act. Alexios Strategopoulos took the city two years later.
The consequence for Greece was more durable. William II spent three years in captivity and bought his freedom in 1262 by ceding the fortresses of Mystras, Monemvasia and Maina in the south-eastern Morea.
Those three castles became the nucleus of Byzantine power in the Peloponnese and grew into the Despotate of the Morea, which outlasted Constantinople itself by seven years — falling to the Ottomans in 1460.
Pelagonia decided which Greek state would restore the empire. Epirus had a claim as good as Nicaea's and had held Thessalonica and an imperial title of its own thirty years earlier; after 1259 it was a regional power and nothing more.
It also set the shape of the following two centuries in Greece. The Franks of the Morea never recovered their dominance, the Byzantines had a foothold at Mystras that became a capital and a centre of late Byzantine culture, and the two coexisted in the Peloponnese until the Ottomans removed both.
And it made the restoration possible without making it secure. Michael VIII got Constantinople because of Pelagonia, and spent the rest of his reign defending it against the western powers whose army he had destroyed here.