William ruled Achaea as regent for his brother during Geoffrey's military campaigns against the Greeks of Nicaea, who were the principal enemies of his overlord, the Latin Emperor of Constantinople Baldwin II.

He participated in the unsuccessful Egyptian crusade of Louis IX of France, who rewarded him with the right to issue currency in the style of French royal coins.

A succession crisis in Nicaea prompted the Epirote ruler Michael II Komnenos Doukas to form an anti-Nicaean coalition with William and Manfred of Sicily.

As his ally soon died, Geoffrey approached Boniface of Montferrat, the ruler of the newly established Frankish Kingdom of Thessalonica, for military aid.

[15] He laid siege to Monemvasia with the support of a Venetian fleet and other Frankish rulers, including Guy I de la Roche, Lord of Athens (who owed allegiance to William for the Moreot fiefs of Argos and Nauplia), and Angelo Sanudo, Duke of the Archipelago.

[15] First, in late 1248 and early 1249, he personally directed the construction of Mistra near the Mount Taygetus; then the castle of Grand Magne was built on the Laconian Gulf.

[15] These castles secured the Frankish control of the Mount Taygetus, forcing the local Slavic tribe of the Melingoi to acknowledge William as their ruler in return for the confirmation of their liberty.

[29] As the triarchs owed allegiance to both Achaea and Venice, Grapella could cite a 1216 ruling by the Venetian bailo, or governor, of Negroponte, that stated a co-ruler of a triarchy was entitled to re-unite it if his or her partner died without issue.

[39] 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.

[41] The ruler of Epirus, Michael II Komnenos Doukas, wanted to take advantage of the Nicaean power struggles by fostering a wide anti-Nicaean coalition.

[48] After the Frankish and Epirote commanders decided to fight a pitched battle instead of attacking fortified towns, the allies marched to Macedonia as far as the plain of Pelagonia to meet the enemy force in June 1259.

[50] As their constant attacks exhausted the Franks and Epirotes, Michael II entered into negotiations with John Palaiologos's envoys, who urged him to desert his Frankish allies.

The Byzantine historian, George Pachymeres, asserts that the Epirote–Frankish coalition split after Achaean knights disrespected the beautiful Vlach wife of Michael II's bastard son, John Doukas, because William refused to discipline them.

Outraged by William's rude remarks about his illegitimate birth, John Doukas deserted to the Nicaeans and convinced his father to abandon the campaign.

[54] William refused, stating that Achaea was "a land acquired by force of arms, held by right of conquest" by the conquerors' descendants, and that he could not surrender his vassals' territory.

He concluded an alliance with Genoa to secure naval support for the siege, but his general, Alexios Strategopoulos, seized Constantinople without Genoese assistance, taking advantage of the absence of the Latin garrison on 25 July 1261.

[54] As they had been built or conquered by William, their transfer did not violate Achaean customary law, but he could not cede frontier castles "without the counsel and consent of his liegemen".

[60] As a consequence of the Byzantine expansion in the Morea, William could rarely offer fiefs to western European knights, which diminished his principality's military power.

[60] Pope Urban IV urged the Catholic bishops and abbots of Frankish Greece to support the Achaeans against the "schismatic" Byzantines on 27 April 1263, but he soon realized he could achieve his principal goal, the unity of Christendom, only through negotiating with Michael VIII.

[60] In response to the Byzantine offensive in the Morea, Urban IV proclaimed a crusade against Michael VIII, but he also appointed new delegates to start negotiations with him about the church union.

[54] Instead of promoting an anti-Byzantine coalition, the papal delegates mediated a reconciliation between Michael VIII and William before the Pope died in October 1264.

[65] Urban's successor, Pope Clement IV, granted the Kingdom of Sicily to King Louis IX's ambitious younger brother, Charles I of Anjou in 1265.

In return, Charles promised military support to recover the territory lost to the Byzantines, but the details of his assistance remained unclarified.

Baldwin II confirmed the treaty and ceremoniously ratified in the presence of the Pope, 14 cardinals and further ecclesiastic and secular dignitaries in the papal chamber in Viterbo on 24 May 1267.

[70] On his return to Achaea in January 1269, William captured Valona to establish a secure bridgehead for Charles's troops on the eastern shore of the Adriatic Sea.

After her return from Constantinople, Margaret revived her claim to Akova and took the influential aristocrat John of Saint Omer as her husband to secure his support.

[15] The author Nicolas Cheetham writes William was ambitious "and headstrong by nature", but his "political projects, conceived on a grand scale, were apt to fail because he over-estimated his strength and took too many risks".

[54] Although the Byzantine governors of the Morea conquered the baronies of Passavant and Kalavryta during the last years of his reign, William kept most lands he had inherited from his brother.

As Setton writes, she "comes to life in the pages of Frankish history only on the day she died, for her death caused the war of the Euboeote succession".

Her claim gave rise to civil war between the supporters of her niece Matilda of Hainaut and her son-in-law Ferdinand of Majorca after she died in 1315.