A standard supply function: stand in line, and the man is issued a weapon, signing for it of course.;
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The U.S used no nuclear weapons whatsoever on Vietnam.
More powerful weapons
I don't think anyone did.
Fear that the North Vietnamese would obtain nuclear weapons themselves (from Russia or China) and use them on South Vietnam or even on America.
The US did not want to risk a nuclear confrontation with the communist superpowers.