Final answer:
Détente was the key policy that helped prevent a devastating nuclear war by promoting cooperation between the US and the Soviet Union while still engaging in competition.
Explanation:
Détente was the policy that substantially reduced the chances of a devastating nuclear war during the late 1960s and early 1970s. Under détente, the United States and the Soviet Union aimed to cooperate while still being in competition and conflict, leading to agreements like SALT I and SALT II that limited nuclear arms.
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