"...advantages that a centrally-located base of power in Hue provided in combating the French also created a strategic conundrum for the rebel forces - did it make more sense to thrust north, towards Hanoi, or south, towards Saigon, from their captured port at Da Nang? It was a fateful decision on which the history of Vietnam would revolve to go for the latter, as Tran Cao Van advocated and Emperor Duy Tan eventually acceded to, overruling the advice of Cuong De to press their advantage in Tonkin, which was less sympathetic to the French than increasingly Catholic Cochinchina and which the French were less emotionally attached to than Saigon, their beloved "Paris of the Orient." Cuong De was under no illusion that in one stroke the French could be entirely driven from Indochina, but his strategy was to link up with Chau's Quang Phuc revolutionary paramilitaries in the western hills of Tonkin and then hope that Chinese paramilitaries of the Kuomintang would cross the border as mercenaries or fellow travelers, thus making a retaking of the whole of Vietnam a hugely arduous undertaking.
Tran Cao Van opposed this as he did not trust Chau, the Quang Phuc, or indeed a great many of the Tonkinese, viewing them as too favorable towards the Chinese and too republican; he was a mandarin through and through. In fairness to Van, however, his position also did have a strategic component to it - Rollet's forces were rapidly advancing up the coast towards Quang Nam Province, a particularly restive province which the rebels had just secured (and, not coincidentally, Van's place of birth), and he argued that the rebels could depend on the Quang Phuc to keep French forces in Tonkin occupied long enough for Duy Tan's army to meet Rollet in the field and either via tactical draw or defeat in the field, force a strategic retreat by the Foreign Legion back towards Saigon. There was a certain sense to it - a two-front war would be difficult enough, and Rollet was the immediate threat.
Two major events occurred in June 1917 that crippled the rebel efforts. The first was a declaration by France of a total blockade of all Indochinese ports with the exception of French military vessels or French-flagged merchants, and the entirety of the French Oriental Fleet was deployed to the waters of the South China Sea to enforce it. Smuggling vessels from China or Japan had thus only a few weeks of window to bring supplies into Vietnam; rifles, bullets and bombs would now have to stream in overland, far from where Duy Tan's forces needed them, and contrary to what the French largely believed at the time, the Germans enforced a strict effort to prevent contraband from moving over the Cambodia borders as best they could, trying to avoid setting a precedent of one European power fomenting a colonial rebellion against another.
The second was the defeat by the Indochinese forces under French command of Quang Phuc battalions at Vinh Yen on June 10, 1917, northwest of Hanoi, which saw nearly ten thousand rebels killed in the battle itself and an additional four thousand captured who were executed shortly thereafter. While both Chau and Hien escaped the slaughter north towards the Chinese frontier, a second battle days later at Son Tay saw rebels pushed further back into the hills, ending the immediate threat to Hanoi. Had Cuong De's position won the day, there could have perhaps been enough pressure on French forces in Tonkin to watch their southern flank to avoid such routs, though modern historians have laid doubt on the idea that Duy Tan's faction could have marched sufficiently close to Tonkin to be a strategic consideration for the French.
As such, it all came down then to the faceoff with Rollet, with Tonkin for the time being pacified. Rollet's army was a motley mix of Foreign Legionnaires augmented by colonial troops almost exclusively drawn from Formosa and Hainan who had little to no reason to feel loyalty to Vietnam, as well as a division of Algerian zouaves known as some of the most ferocious fighters in the French Army. Van was a wily political operator but not particularly talented at tactics, and sought to maneuver his men through the Central Highlands as much as possible to enjoy good high ground, delaying his confrontation with Rollet repeatedly while the French commander camped his men in Quang Ngai and secured the mountains, and mountain passes, immediately to its west and southwest. Van eventually cracked and elected to attack Rollet, ceding the decision to fight on ground of his choosing and pushing ahead towards the enemy.
Rollet had picked his place of battle carefully. He had arrived there from Cam Ranh aware that it was easily resupplied from Quy Nhon, and that by staying south of the Tra Kuc River he had an excellent defensive position despite the city's flat geography; the Tra Kuc was not nearly the same kind of marshy delta found at Da Nang, and during the dry season he had little to concern himself about vis a vis flooding. Mobile light artillery had been moved up into the mountains on his flank, and Rollet became the first Foreign Legionary to incorporate air support into his arsenal after reading reports on its effectiveness in the Great American War, with eight CASD two-seater strafing planes and three bomber-fighters based out of a makeshift airfield in a drained rice paddy to his south. Van's stalling had given Rollet time to prepare, and the result was a bloodbath. The Battle of Quang Ngai saw the bulk of Duy Tan's army defeated as it tried to cross the river, and while the limits of air superiority in colonial conflicts would become apparent in future European operations in places like Africa in the decades to come, in 1917 it was an almost decisive advantage over an army of Vietnamese peasants. Five thousand died, another ten thousand surrendered (and unlike commanders in the north, the famously brutal Rollet elected to throw them in prison camps rather than carry out mass executions, though considering the squalor of said camps it was perhaps a distinction without a difference), and thousands more were scattered into the hills. The May Rebellion did not end on July 1st, 1917, but it might well have.
Van was captured days later by Rollet's scouts and summarily executed as one of the chief leaders of the rebellion, essentially breaking the forces in the center of Vietnam, and Rollet steadily marched northwards towards Da Nang throughout July. Duy Tan here made one of the most difficult decisions of his life - seeing that the rebellion had, almost certainly, failed, he elected to instead call upon the Vietnamese to throw down their arms, and made arrangements to surrender to Rollet in Da Nang and with that formally end the conflict. Formally is the key word, here - Duy Tan fell on his sword in large part to give many of the rebellion's other leaders a chance to flee west, in this case into northern Cambodia and eventually to Phnom Penh or, in the case of Cuong De, to Bangkok, where he became the leader of a prominent exile community that continued to aggressively advocate the overthrow of the French.
Rollet and Sarraut together accepted Duy Tan's surrender on the deck of the aging battleship Charles Martel, a ship which would whisk him and several courtiers away immediately into exile with his father on Reunion. Not long thereafter, a cousin, Khai Dinh, was proclaimed the new Emperor of Vietnam, a choice that could not have been more obviously a unilateral decision by Paris to appoint a stooge to the throne had it been telegraphed as such in Vietnamese on every street corner. The Vietnamese intelligentsia was appalled, as were the average commoners, and Khai Dinh rapidly became among the most unpopular figures in Vietnamese history. He lived opulently in luxury financed by the French while most of his subjects lived in acute poverty and had their taxes raised repeatedly for his vanity projects; he was allegedly a homosexual, and he signed a raft of death warrants for nationalist leaders such as Chau, Tien and others that kept them in exile for years.
The May Rebellion's failures nonetheless proved ominous for the French; they had successfully put down a full-fledged colonial rebellion within the space of months, but they had done so in a fashion that left the polity embittered, and there were now sophisticated revolutionary and anti-colonial cells in Canton and Bangkok, respectively, who could organize and regroup, and a national hero in Duy Tan that Rollet had been keen enough to see would be a permanent martyr had he been killed. The battle was over, but for the cause of Vietnamese independence, the war was far from lost..."
- The French Orient