Which nation did the director fear might use the weapon they had developed against the soviet union

During Cold War(1945-91),USA had the fear to use the nucleic weapon they developed against the Soviet Union(USSR).

As USSR had developed nucleic weapon as well,which could cause extreme casualties.For instance,an entire city could be easily destroyed,not only would it cause economic loss,the world would be at threat.

Clearly knowing that once they launch a nucleic weapon, and a nuclear war would burst out,two nations both tried their best to avoid using the weapon.

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