With Beijing's promise of limited assistance to Hanoi, the communist military strategy concentrated on the liberation of Tonkin and consigned Cochinchina to a lower priority. The top military priority, as set by Giap, was to free the northern border areas in order to protect the movement of supplies and personnel from China.
By autumn of 1950, the Viet Minh had again liberated the Viet Bac in decisive battles that forced the French to evacuate the entire border region, leaving behind a large quantity of ammunition.
From their liberated zone in the northern border area, the Viet Minh were free to make raids into the Red River Delta. The French military in Vietnam found it increasingly difficult to convince Paris and the French electorate to give them the manpower and materiel needed to defeat the Viet Minh.
For the next two years, the Viet Minh, well aware of the growing disillusionment of the French people with Indochina, concentrated its efforts on wearing down the French military by attacking its weakest outposts and by maximizing the physical distance between engagements to disperse French forces. Being able to choose the time and place for such engagements gave the guerrillas a decided advantage. Meanwhile, political activity was increased until, by late 1952, more than half the villages of the Red River Delta were under Viet Minh control.
The newly appointed commander of French forces in Vietnam, General Henri Navarre, decided soon after his arrival in Vietnam that it was essential to halt a Viet Minh offensive underway in neighboring Laos. To do so, Navarre believed it was necessary for the French to capture and hold the town of Dien Bien Phu, sixteen kilometers from the Laotian border. For the Viet Minh, control of Dien Bien Phu was an important link in the supply route from China. In November 1953, the French occupied the town with paratroop battalions and began reinforcing it with units from the French military post at nearby Lai Chau.
During that same month, Ho indicated that the DRV was willing to examine French proposals for a diplomatic settlement announced the month before. In February 1954, a peace conference to settle the Korean and Indochinese conflicts was set for April in Geneva, and negotiations in Indochina were scheduled to begin on May 8. Viet Minh strategists, led by Giap, concluded that a successful attack on a French fortified camp, timed to coincide with the peace talks, would give Hanoi the necessary leverage for a successful conclusion of the negotiations.
Accordingly, the siege of Dien Bien Phu began on March 13, by which time the Viet Minh had concentrated nearly 50,000 regular troops, 55,000 support troops, and almost 100,000 transport workers in the area. Chinese aid, consisting mainly of ammunition, petroleum, and some large artillery pieces carried a distance of 350 kilometers from the Chinese border, reached 1,500 tons per month by early 1954. The French garrison of 15,000, which depended on supply by air, was cut off by March 27, when the Viet Minh artillery succeeded in making the airfield unusable. An elaborate system of tunnels dug in the mountainsides enabled the Viet Minh to protect its artillery pieces by continually moving them to prevent discovery. Several hundred kilometers of trenches permitted the attackers to move progressively closer to the French encampment. In the final battle, human wave assaults were used to take the perimeter defenses, which yielded defensive guns that were then turned on the main encampment. The French garrison surrendered on May 7, ending the siege that had cost the lives of about 25,000 Vietnamese and more than 1,500 French troops.
The following day, peace talks on Indochina began in Geneva, attended by the DRV, the Associated State of Vietnam, Cambodia, Laos, France, Britain, China, the Soviet Union, and the United States. In July a compromise agreement was reached consisting of two documents: a cease-fire and a final declaration. The ceasefire agreement, which was signed only by France and the DRV, established a provisional military demarcation line at about the 17°N parallel and required the regroupment of all French military forces south of that line and of all Viet Minh military forces north of the line. A demilitarized zone (DMZ), no more than five kilometers wide, was established on either side of the demarcation line.
The cease-fire agreement also provided for a 300-day period, during which all civilians were free to move from one zone to the other, and an International Control Commission, consisting of Canada, India, and Poland, to supervise the ceasefire. The final declaration was endorsed through recorded oral assent by the DRV, France, Britain, China, and the Soviet Union. It provided for the holding of national elections in July 1956, under the supervision of the International Control Commission, and stated that the military demarcation line was provisional and "should not in any way be interpreted as constituting a political territorial boundary." Both the United States and the Associated State of Vietnam, which France had recognized on June 4 as a "fully independent and sovereign state," refused to approve the final declaration and submitted separate declarations stating their reservations.