He was the Preceptor of Aquitaine in France for the Templars[1] and arrived in the Holy Land around autumn of 1247, finding "the remnants of the Kingdom of Jerusalem in a precarious state.

On 5 June 1249, the French crusader army, combined with de Sonnac and his Templar knights, attempted to land in Egypt.

The next day the de Sonnac wrote to Robert of Sandford, telling how on the morning after the battle, Damietta had been seized with only one crusader casualty.

De Sonnac's next engagement was at the Battle of Mansurah, for the city containing the area's defensive force, the last obstacle to central Egypt.

[1] De Sonnac, Robert of Artois, the King's brother and William Longespee, leader of the English troops, launched an assault on the Muslim force without the main Frankish army.

Taken by surprise, the Egyptians quickly retreated from the riverbank into the city and the Count foolishly gave chase, outnumbered and with no back-up from the bulk of the French forces.

Robert was "bellowing and swearing disgracefully as is the French custom",[4] and blaming the Templars and other religious orders for causing the real downfall of the Kingdom.

A major assault was launched by the Muslims on 6 April and de Sonnac joined the Frankish charge to meet the enemy.

And you should know that there was at least an acre of land behind the Templars, which was so covered with arrows fired by the Saracens, that none of the ground could be seen[3]His death and the capture of Louis IX marked an end to the seventh crusade.

[5] Guillaume de Sonnac is a prominent character in Crusades, a graphic novel by Izu, Alex Nikolavitch, and Zhang Xiaoyu, published by Les Humanoïdes Associés, in which he is shown as a power hungry plotter.