Answer:
One major factor that led to the instability of the government of South Vietnam was the toppling of it president Ngo Dinh Diem and the execution of both himself and his brother Ngo Dinh Nhu.
This happened during a coup staged by some of his generals. It is possible that the fact that the US presence was becoming too intolerable for certain factions.
In the 1950s before the incidence leading up to the death of Diem, US troops were no more than 800 in that region.
Because the US wanted to secure her interests in the Southeast Asian vicinity, her strategy was to ensure that Diem remained in power. So they supported Diem with aid to help him overcome Viet Cong who at that time was a threat to Diem's government.
Even after the Vietnamization, a program initiated by the US to limit going forward the loss of American soldiers and to better equip the Vietnamese proved abortive. Sporadic bursts of violence and large scale destruction still plagues South Vietnam
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