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.