(i) The war between Vietnam and US grew out of fear among US policy planners that the victory of the Ho Chi Minh government would start a domino effect-communist governments would be established in other countries in the area.
(ii) They underestimated the power of nationalism to move people to action, inspire them to sacrifice their home and family, live under horrific conditions and fight for independence.
(iii) They underestimated the power of a small country to fight the most technologically advanced country in the world.