the two triarchs were indeed incarcerated, they were held in captivity only for some months. William appointed his nephew Geoffrey of Briel to lead an army to Negroponte. The Achaean troops laid waste to the island and expelled the Venetians. The Doge of Venice, Reniero Zeno, made Marco Gradenigo the new . Gradenigo gained the support of William's vassal, William de la Roche and his brother, Guy I of Athens. Guglielmo of Verona and Narzotto dalle Carceri met Gradenigo at Guy's seat in Thebes on 14 June 1256. At the meeting, the two triarchs rejected Achaean overlordship and swore fealty to Venice for their lordships. Two other Frankish lords, Thomas II d'Autremencourt, Lord of Salona and Ubertino Pallavicini, Marquess of Bodonitsa, joined the anti-Achaean coalition, while William secured the support of Othon de Cicon, Lord of Karystos in Negroponte and the Genoese. Gradenigo attacked Chalcis, and the Venetian infantrymen routed the Achaean cavalry near the city. The war quickly spread to mainland Greece. When William made preparations for an invasion of Attica, Geoffrey of Briel, who was the son-in-law of Guy of Athens, deserted him. Before long, Chalcis surrendered to the Venetians. William assembled his army and invaded Athens across the Isthmus of Corinth in the spring of 1258. Guy I of Athens and his allies tried to stop the invasion at Mount Karydi, but William inflicted a decisive defeat on them in May 1258. Guy and his allies fled to Thebes, and William pillaged Attica and Boeotia. William attacked Thebes, but the city's archbishop persuaded him to abandon the siege. Guy pledged he would never fight against William and agreed to accept the decision of the High Court of Achaea for his disloyalty. After hearing Guy at Nikli, the Achaean aristocrats sitting at the High Court decided they could not judge the case because Guy owed fealty to William only for his Moreot domains, not for the Lordship of Athens. Instead of forfeiting Guy's Moreot fiefs, the High Court referred his case for judgement to King Louis IX. As Guy accepted this ruling, peace was restored between the two Frankish rulers. Louis IX received Guy in Paris in June 1259. The King and the French barons decided Guy had done adequate penance for breaking his oath of fealty by undertaking the arduous journey to France. Defeat and captivity Doge Ranieri Zeno ordered the new of Negroponte, Andrea Barozzi, to