Trương Định (1820 – August 19, 1864), sometimes known as Trương Công Định, was a mandarin (scholar-official) in the Nguyễn dynasty of Vietnam under Emperor Tự Đức. He is best known for leading a guerrilla army in southern Vietnam against French forces in defiance of the emperor. He refused to recognise the 1862 Treaty of Saigon that ceded Vietnamese territory to France. The son of a military mandarin from central Vietnam, Định moved south when his father was posted to Gia Định as the provincial commander. Định grew up to lead a military colony, overseeing the settlement and economic development of his constituency. He gained a reputation for being an able leader and land developer who cared for his people. When France began its invasion of southern Vietnam in 1859, Định organised local militia to reinforce the imperial army. As the regular army units suffered defeats on the battlefield, its remnants joined Định's partisans, and by 1861, he had around 6,000 men under his command. Định built his own resistance base and organised guerrilla attacks against the French. His success led Tự Đức to grant Định command of the southern partisans and to order the regular commanders to coordinate their plans with him. Định's forces quickly gained the respect of the French. They focused on disrupting the French bureaucracy and military posts in addition to the transportation of rice. Their most notable attack was the sinking of L'Espérance in December 1861. Amid growing military setbacks, Vietnam signed the Treaty of Saigon in June 1862, losing three southern provinces which became the French colony of Cochinchina. Tự Đức hoped that by abiding by the treaty, Vietnam could eventually negotiate the future return of the territory. He ordered the partisans to disband, but Định refused to recognise the treaty and disobeyed the monarch, fighting on in defense of his homeland. With the loss of imperial support, Định's forces began to struggle against the superior resources of the French. His men were gradually worn down and, following an ambush, Định committed suicide to avoid being captured. Vietnamese and French historians have fiercely debated Định's choice to disregard the treaty. The effectiveness of Định's insurgency led French officials to claim that Tự Đức was secretly assisting him, thereby violating the treaty, a pretext the French used to seize three more provinces in 1867. Vietnamese records dispute this, contending that Tự Đức attempted to ensure