Complex sentence: During the First Indochina War Le Duan served as the Secretary of the Regional Committee of South Vietnam, at first in Cochin China in 1946, but was reassigned to head the Central Office of South Vietnam from 1951 until 1954.
Split into simpler sentences: During the First Indochina War Le Duan served as the Secretary of the Regional Committee of South Vietnam. At first in Cochin China in 1946, but he was appointed to be responsible for the Central Office of South Vietnam from 1951 until 1954.