During the Cold War, before the day of the ABM treaty, the situation between the United States and the Soviet Union was extremely dangerous, as each country was coming close to acquiring what was called “First strike capability”. If you had first strike capability it meant that you could launch a surprise “first strike” on the enemy which would be so devastating that they could do little harm to you in retaliating. If the your enemy had first strike capability, that would mean that they could launch a surprise attack against you that would be so devastating that you could not effectively retaliate.