A conflict fought between French colonial forces and Vietminh forces largely in the Tonkin area of northern Vietnam. The Vietminh began active guerrilla operations during the Japanese occupation of World War II and in September 1945 their leader, Ho Chi Minh, proclaimed a Vietnamese Republic in Hanoi. The French opposed independence, and launched a military offensive. Ho Chi Minh was forced to flee Hanoi and begin a guerrilla war in December 1946. By 1950, foreign communist aid had increased Vietminh strength to the point where the French were forced into defensive lines around the Red River delta, but Vietminh attempts to win the war failed in 1951. Guerrilla operations continued until an ill-advised French attempt to seek a decisive engagement led to the encirclement and defeat of their forces at Dienbienphu in 1954. The war, and French rule in Indo-China, were formally terminated at the Geneva Conference in April–July of that year.