For decades, the threat of nuclear conflict between the U.S. and the Soviet Union hung over humanity — and occasionally the superpowers edged toward the brink, as with the Cuban missile crisis.

But beginning in the 1970s, American and Soviet leaders started taking steps toward de-escalation, leading to a handful of critical treaties, including the 1987 Intermediate-range Nuclear Forces Treaty that eliminated an entire class of nuclear-capable missiles.

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