Question: what did the second largest city by population in vietnam also serve as in the first indochina war?

Documents:
source: Hanoi
text: Hanoi Hanoi ( or ; Vietnamese: "Hà Nội" , ] ) is the capital of the Socialist Republic of Vietnam and the country's second largest city by population. Its population in 2009 was estimated at 2.6 million for urban districts and 7 million for the metropolitan jurisdiction. The population in 2015 was estimated at 7.7 million people. From 1010 until 1802, it was the most important political centre of Vietnam. It was eclipsed by Huế, the imperial capital of Vietnam during the Nguyễn Dynasty (1802–1945), but Hanoi served as the capital of French Indochina from 1902 to 1954. From 1954 to 1976, it was the capital of North Vietnam, and it became the capital of a reunified Vietnam in 1976, after the North's victory in the Vietnam War.

source: Việt Bắc
text: Việt Bắc Việt Bắc ("Northern Vietnam") is a region of Vietnam north of Hanoi that served as the Viet Minh's base of support during the First Indochina War (1946–1954).

source: First Indochina War
text: First Indochina War The First Indochina War (generally known as the Indochina War in France, and as the Anti-French Resistance War in Vietnam) began in French Indochina on 19 December, 1946, and lasted until 1 August, 1954. Fighting between French forces and their Viet Minh opponents in the south dated from September 1945. The conflict pitted a range of forces, including the French Union's French Far East Expeditionary Corps, led by France and supported by Emperor Bảo Đại's Vietnamese National Army against the Viet Minh, led by Ho Chi Minh and the People's Army of Vietnam led by Vo Nguyen Giap. Most of the fighting took place in Tonkin in northern Vietnam, although the conflict engulfed the entire country and also extended into the neighboring French Indochina protectorates of Laos and Cambodia.

source: Dien Bien Phu (film)
text: Dien Bien Phu (film) Diên Biên Phu (French for "Điện Biên Phủ") is a 1992 film written and directed by French veteran Pierre Schoendoerffer. With its huge budget, all-star cast, and realistic war scenes produced with the cooperation of the French and Vietnamese armies, "Dîen Bîen Phu" is regarded by many as one of the more important war movies produced in French filmmaking history. It portrays the 55-day siege of Dien Bien Phu (1954), the last battle by the French Union's colonial army in the First Indochina War during the final days of French Indochina, which was soon after divided into North and South Vietnam. This was a prelude to the Second Indochina War, known in the United States as the Vietnam War.

source: Bernard de Lattre de Tassigny
text: Bernard de Lattre de Tassigny Bernard de Lattre de Tassigny (11 February 1928 – 30 May 1951) was a French Army officer, who fought during World War II and the First Indochina War. Bernard de Lattre received several medals during his military career, including the Médaille militaire. He was killed in action at the age of 23, fighting near Ninh Binh. At the time of his death, his father, General Jean de Lattre de Tassigny, was the overall commander of French forces in Indochina. Bernard's death received widespread newspaper coverage, with headlines drawing attention to the death of the son of a general. His mother worked to preserve the memory of her son, as well as that of her more famous husband who died in 1952. Their legacy includes an open-air memorial chapel and centre in Wildenstein, Alsace, France. The death of Bernard de Lattre is mentioned in histories of the First Indochina War, and it has been compared to the deaths of other sons of generals and military leaders.

source: List of war museums and monuments in Vietnam
text: List of war museums and monuments in Vietnam There are numerous war museums, memorials and monuments in Vietnam, this page presents a partial list of museums and monuments located in Vietnam relating to the First Indochina War and the Second Indochina War. This list is organized by location.

source: Geneva Agreements
text: Geneva Agreements The Geneva Agreements of 1954 (also, "Geneva Accords") arranged a settlement which brought about an end to the First Indochina War. The agreement was reached at the end of the Geneva Conference. A ceasefire was signed and France agreed to withdraw its troops from the region. French Indochina was split into three countries: Laos, Cambodia, and Vietnam. Vietnam was to be temporarily divided along the 17th Parallel until elections could be held to unite the country. These elections were never held; following repeated refusals to hold nationwide elections by Ngo Dinh Diem and his declaration of leadership of a new state, South Vietnam, the Vietminh established a communist state in the North led by Ho Chi Minh. The US gave Diem considerable support in the form of financial aid; due to the corruption evident in his regime, and the question of the depth of support for him in Vietnam, there was a certain amount of reluctance in doing so.

source: Landmines in the Vietnam War
text: Landmines in the Vietnam War Since the outbreak of the First Indochina War in 1946 and later the bloodier Second Indochina War of the 1960s and 1970s, countless numbers of land mines have been planted in what is now the Socialist Republic of Vietnam. Many of these devices that did not detonate at some point or another remain a very dangerous menace that continues plaguing the country and surrounding areas.

source: Patrouille de choc
text: Patrouille de choc Patrouille de choc, English title Shock Patrol is a 1957 French war film set during the First Indochina War that was written and directed by Claude Ogrel under the name Claude Bernard-Aubert. Ogrel was a war correspondent in French Indochina from 1949-1954 and this was his film debut. The film was the first French film about the First Indochina War. The original title "Patrouille sans espoir" ("Patrol Without Hope") was changed along with the original pessimistic ending.

source: Vietnam Campaign Medal
text: Vietnam Campaign Medal The Republic of Vietnam Campaign Medal also known as the "Vietnam Campaign Medal" (Vietnamese: "Chiến Dịch Bội Tinh" ) is a military campaign medal which was created in 1949, and awarded to French military personnel during the First Indochina War. During the Vietnam War (Second Indochina War), the South Vietnamese government awarded the Republic of Vietnam Campaign Medal with Device ("1960- ") to members of the South Vietnamese military for wartime service and on March 24, 1966, to members of the U.S. military for support of operations in Vietnam. In May 1966, other allied foreign military personnel became eligible for the award.

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Answer: Viet Minh's base