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To estimate the deterrent effect on the Viet Minh of certain US courses of action; to assess the consequences of a US failure to counter overt Viet Minh aggression and to assess the political repercussions of US armed intervention against Viet Minh aggression.

I. INTRODUCTORY NOTE

1. We continue to regard as valid the estimate made in NIE 63.1-55, namely that during the period of the estimate (to July 1956) "concern for Western and particularly US reactions, together with general considerations arising from over-all Bloc policy, will prevent the DRV from openly invading the South." We also believe that the present Communist policy of reducing international tension makes unlikely the resort to overt aggression.

2. This assumed US course of action would render even less likely than at present any overt aggression by the Viet Minh against South Vietnam. The Communists would probably estimate that: (a) Viet Minh forces alone would not be able to capture South Vietnam in the face of swift and determined US armed intervention; (b) Chinese Communist assistance would have to be on such a scale as to seriously risk spreading the war beyond Vietnam; and (c) acquisition of South Vietnam would not be worth such a risk. These considerations would lead the Communists to refrain from overt aggression even if they could see no prospect of winning South Vietnam by other means and believed they enjoyed a marked superiority in forces locally available.

3. The assumed US course of action would not in itself preclude either a Communist decision to initiate large-scale guerrilla action in South Vietnam or the clandestine support of such actions by the Viet Minh.

4. The deterrent effect on the Communists of the assumed course of action would be in-increasedincreased [sic] if the US made it clear that nuclear weapons would be used. If the US made it clear that nuclear weapons would not be used the deterrent effect would be reduced. However, the Communists would probably estimate that the US would not maintain such a position if faced with a deteriorating military situation. In the absence of any clear indication, the Communists would almost certainly base their plans on the possibility that nuclear weapons would be used.

Rh