William I de la Roche succeeded his brother, John I

John I de la Roche

John I de la Roche succeeded his father, Guy I, as Duke of Athens in 1263. He was cultured and chivalrous, spoke fluent Greek, and read Herodotus.In 1275, John, with 300 knights, relieved Neopatras, blockaded by a Byzantine mercenary army...

, as Duke of Athens in 1280. He was the first official "duke" of Athens; previous dukes had actually been "lords."

William reversed the territorial losses of his brother's reign, extending his control over Lamia

Lamia (city)

Lamia is a city in central Greece. The city has a continuous history since antiquity, and is today the capital of the regional unit of Phthiotis and of the Central Greece region .-Name:...

 and Gardiki

Gardiki

Gardiki , older form: Gardikion may refer to the following villages with of including this name in Greece:*Gardiki Omilaion, a village in Phthiotis*Gardiki Souli, a village in Thesprotia, part of Souli...

 as far as Argos and Nauplia

Argos and Nauplia

During the late Middle Ages, the two cities of Argos and Nauplia formed a separate Lordship within the Frankish Principality of Achaea in southern Greece....

. He married Helena Angelina Komnene, daughter of John I Doukas

John I Doukas

John I Doukas was ruler of Thessaly from c. 1268 to his death in 1289....

, ruler of Thessaly, securing a military alliance with him.

In 1285, while Charles II of Naples

Charles II of Naples

Charles II, known as "the Lame" was King of Naples, King of Albania, Prince of Salerno, Prince of Achaea and Count of Anjou.-Biography:...

, nominal prince of Achaea, was imprisoned, Robert of Artois, regent of the kingdom, named William bailiff and vicar-general of Achaea. He built the castle of Dimatra to defend Messenia

Messenia

Messenia is a regional unit in the southwestern part of the Peloponnese region, one of 13 regions into which Greece has been divided by the Kallikratis plan, implemented 1 January 2011...

 from the Byzantine Empire

Byzantine Empire

The Byzantine Empire was the Eastern Roman Empire during the periods of Late Antiquity and the Middle Ages, centred on the capital of Constantinople. Known simply as the Roman Empire or Romania to its inhabitants and neighbours, the Empire was the direct continuation of the Ancient Roman State...

. He was then the most powerful baron in Frankish Greece. In 1286, he arbitrated the succession of the March of Bodonitsa following the death of Isabella Pallavicini

Isabella Pallavicini

Isabella Pallavicini , sometimes Jezebel, was the marchioness of Bodonitsa from 1278. She succeeded her brother Ubertino and also inherited her elder sister Mabilia's Italian possessions in Parma. The three were the only children of the first margrave Guy...

. He chose her cousin Thomas

Thomas Pallavicini

Thomas Pallavicini was the margrave of Bodonitsa following a disputed succession in 1286. He was the grandson of Rubino, younger brother of Guy, the first margrave....

 over her widower Antoine le Flamenc

Antoine le Flamenc

Anthony was the Frankish baron of Karditsa . He was married to Isabella Pallavicini and co-ruled the March of Bodonitsa with her from 1278 to 1286, when she died...

.

William's rule was peaceful, but short, as he died two years after assuming power in Achaea. He was succeeded by his minor son Guy

Guy II de la Roche

Guy II de la Roche was the Duke of Athens from 1287, the last duke of his family. He succeeded as a minor on the death of his father, William I, at a time when the duchy of Athens had exceeded the Principality of Achaea in wealth, power, and importance.Guy was originally under the tutorship and...

.