French colonial administration lasted until March 9 , 1945 , during Japanese occupation ( 1941 – 1945 ) . Although French administration was allowed during Japanese occupation as a puppet government , Japan briefly took full control of Vietnam in March 1945 under the Empire of Vietnam and Tonkin became the site of the Vietnamese Famine of 1945 during this period . At the end of the war , the north of Vietnam ( including Tonkin ) saw a sphere of influence by China while the south was briefly occupied by the British for French forces to regroup and regain control . Harry Truman at the Potsdam Conference , stated an intention to hand the region back to French rule , a sharp contrast to Franklin D. Roosevelt 's strong opposition to colonialism and commitment to support the Viet Minh . However , after the Japanese withdrew from Vietnam , Ho Chi Minh proclaimed the establishment of the Democratic Republic of Vietnam in Ba Đình Square . Hanoi was later reoccupied by the French and conflict between the Viet Minh and France broke out into the First Indochina War
Potsdam Conference
Potsdam Conference
Tonkin (French protectorate)