[4][5] Định took advantage of his improved socio-economic status to recruit a group of impoverished people, whom he organised for clearing land and founding a đồn điền ("military colony") in Gò Công.

The missionaries wanted the French administration to facilitate their work converting the Vietnamese to Roman Catholicism, while military and business figures saw strategic and commercial opportunities in Vietnam.

[5] They were armed with bladed spears, bow and arrows, fire lances, knives, machetes, sabres, swords, and a small amount of firearms,[5][6][16] who were trained and ready at whatsoever moment.

As a renowned local leader who was respected for his leadership and military prowess, Định naturally assumed a lead role in the partisan movement that responded to Tự Đức's appeals for popular resistance against the French aggression.

[17] Having fought at Ky Hoa,[18] Định knew he needed more fighters to at least have a fighting chance against the French’s, so he decided to incorporate any soldier who wanted to protect their homeland, from the defeated Southern Army into his ranks.

Who many have been astray after their commander of the fallen Saigon Citadel in 1859, Governor General of Gia Dinh and Bien Hoa Võ Duy Ninh committed suicide after their defeat.

[20] In 1861, the resistance leaders in the Gò Công area delegated Định to travel to Biên Hòa to seek permission from Imperial Military Commissioner Nguyen Ba Nghi to "turn around the situation".

[16] Định's forces began inflicting substantial damage on the European troops, largely because of their intimate knowledge of the terrain, skill in hit-and-run guerrilla tactics, and popular support from the local villagers.

[21] Định's men focused on targeting French soldiers stationed around the countryside and ambushing military installations that weren’t well concentrated as a consequence of their guerrilla pursuit.

[24] At 05:00, 600 rebels under the command of Do Dinh Thoai stormed the military post in the town, engaging the French troops in hand-to-hand combat with lances, bayonets, knives and bamboo sticks.

[22] Định's Gò Công insurgents attempted to stop rice from being shipped to Cholon by attacking the French lorchas and apprehending the trading vessels on the local waterways.

On June 5, 1862, Định broke with the Nguyễn army after the court's plenipotentiary Phan Thanh Giản and another official, Lam Duy Hiep, signed the Treaty of Saigon.

[31] Định then allied himself with Võ Duy Dương to form the Movement of Popular Self Defense (Phong Trao Nhan Dan Tu Ve), creating an operations base in Đồng Tháp.

Claiming that his followers would not allow him to leave, Định refused his appointment to An Giang,[23] instead adopting the title of Bình Tây Sát Tà Đại Tướng (Western Pacifying Antiheresy General).

The slogan "Phan-Lâm mái quốc; Triều đình khi dân" (Phan [Thanh Gian] and Lam [Duy Hiep] sell out the country; the court doesn’t care for the people) was circulated through the region.

He attempted to widen his support base by distributing leaflets as far as the regional centres of Saigon and Mỹ Tho, calling on nghĩa quân from other provinces to join the common struggle.

[36] Quyen was eventually killed and in 1874, long after the southern insurgency had been crushed, Tự Đức granted a monthly allowance in grain and cash to Định's widow Le Thi Thuong, who had returned to her native village in Quảng Ngãi, which at the time was still in independent Vietnamese territory.

[39] The French officers never produced concrete proof of Huế's support for the southern partisans or of the connivance of Vietnamese officials in the sovereign provinces bordering French-occupied territory.

Seeing that it could not withstand a conventional war, the Annamite government organised, openly before the peace, clandestinely and underhandedly afterward, a permanent insurrection in Cochin China.... Quan Dinh [Mandarin Dinh], head of the insurrection at Gò Công, although publicly disavowed by the viceroy of Vĩnh Long Phan Thanh Gian, who has called on him several times to withdraw so that the peace treaty can be implemented, has absolutely refused to do so.

[42] Milton Osborne, while noting that evidence was circumstantial, asserted that French charges were probably legitimate:[42] The scale of the risings in December 1862 certainly suggests an organised concerted effort, backed by Huế.

The imperial records hold that Tự Đức immediately ordered the insurgents to disband, fearing that their actions were an obstacle to his plan of negotiating the return of the lost territory.

[43] In the specific case of Định, the Vietnamese documents record the failed attempts of Gian to persuade him to lay down his arms and accept an administrative post in An Giang.

Định's supporters implored him to disobey the edicts of 1862 that directed the partisans to end hostilities:[45] "Our people forced the Westerners to retreat many times, and now that the court has made peace with them, they will surely kill us....

[45] According to Mark McLeod, it is unclear whether hawkish mandarins in the imperial court were assisting Định's southern insurgents in contravention of Tự Đức's wishes.

[48][49]The historian David Marr agrees, noting that "the distinction quite rightly pointed out by Professor Truong Buu Lam was in all probability bred of immediate adverse conditions and not the product of a long tradition".

In their written confrontation after the signing of the treaty, Gian did not accuse Định of being a rebel, but of excessive devotion to the emperor, which while admirable in principle, was hindering their current strategy.

Although the southern insurgents' disobedience provided the French with a pretext for further aggression, thereby hindering Tự Đức's plans for regaining the lost territory, the emperor never accused them of rebelling against royal authority.

[18]Chiểu further hoped that the Vietnamese court would change its position and come to the aid of the insurgents, writing:[47] "The sigh of the wind and the cry of the crane [announcing the official army's arrival] held you breathless for more than ten months.

In 1964, an article in the North Vietnamese Nghiên cứu lịch sửu described Định as "the hero symbolising the spirit of resistance to the foreign colonialists of the people of Southern Viet-Nam".

[56] Osborne said that Định's final manifesto before his death showed him "to have been a man with a high concept of duty, an awareness of his own weakness and with a sense of despair, common to many of his countrymen, at the ambivalence of the Huế court".