From Embers of War: The Fall of an Empire and the Making of America's Vietnam, by Fredrik Logevall (Random House, 2012), Kindle pp. 183-185:
Ho was in a tough spot, facing pressure from several quarters—from Sainteny and the French, from his Chinese occupiers who counseled moderation, and from Vietnamese nationalist parties (notably the VNQDD and the Dai Viet) who accused him of preparing to sell out to France. The signing of a Sino-French agreement in Chongqing on February 28, in which the Chinese agreed to return home in exchange for significant economic concessions from France, reduced his maneuverability further—the agreement, Ho knew, paved the way for a French invasion of Tonkin.
And indeed, the French were about to launch Operation Bentré, a secret plan for the reoccupation of Indochina north of the sixteenth parallel. Hatched in Leclerc’s headquarters some months earlier (and named for a town and province at the mouth of the Mekong River), the plan had several elements but centered on landing a sizable force at the port city of Haiphong and, in coordination with a smaller force arriving by plane, proceeding to capture Hanoi. Over a period of three days starting on February 27, the French Ninth Division of Colonial Infantry and Second Armored Division—a total force of some twenty-one thousand men, most of them wearing American helmets, packs, fatigues, and boots—boarded warships, and on March 1, a fleet of thirty-five ships sailed from Saigon north along the coast. Because of the movement of the tide, the landing would have to occur on either March 4, 5, or 6, or it could not occur again until the sixteenth. An early objective: to rearm three thousand French soldiers who remained interned at the Hanoi Citadel—and who, Bentré planners surely knew, would be in a vengeance-seeking mood.
The French hoped that the arrival of the troops, following fast on the heels of the Chongqing agreement, would compel Ho to agree to a deal on French terms. But the risks were huge. What if the Vietnamese chose instead to stand and fight? And of more pressing concern, what if the Chinese refused to offer their support to the troop landing? That is what occurred. French general Raoul Salan secured permission from the Chinese to have the vessels “present” themselves in Haiphong’s harbor on March 6 but not to disembark any troops. Chinese leader Chiang Kai-shek, anxious to secure his southern flank at a time when his struggle against Mao Zedong’s Communists was heating up in northeastern China, had no wish to become embroiled in a Vietnamese war of liberation. When the French ships entered the Haiphong harbor on the morning of March 6, the Chinese batteries in the cities began firing. The ships returned fire, and the fighting continued until eleven A.M., with both sides suffering casualties. Chinese negotiators, meanwhile, leaned hard on both the French and the Vietnamese to come to an accord. Strike a bargain, they in effect ordered, or you may find yourselves fighting us as well as your main adversary.
The blackmail tactic worked. In the afternoon of March 6, the two sides, under intense Chinese pressure, signed a “Preliminary Convention,” wherein the French recognized the “Republic of Vietnam” as a “free state” (état libre) within the Indochinese Federation and French Union; the Vietnamese agreed to welcome twenty-five thousand French troops for five years to relieve departing Chinese forces; and France in turn agreed to accept the results of a future popular referendum on the issue of unifying the three regions. The new National Assembly in Hanoi, which had been elected in January, approved the deal, with the understanding that it was preliminary and that additional negotiations would follow in short order. Some Vietnamese militants condemned the accord as a sellout, but Ho reiterated his conviction that the first order of business was to be rid of the dread Chinese. “As for me,” he told aides, “I prefer to sniff French shit for five years than eat Chinese shit for the rest of my life.”