In the two decades since the end of World War Two, both the USA and the USSR had greatly increased their stockpiles of nuclear weapons. The power of these weapons had increased significantly, while delivery methods had grown and improved.
In 1961, the USSR detonated the Tsar Bomba. This nuclear bomb was so powerful that the fireball it produced nearly destroyed the plane that had dropped it. By 1969 the Soviets had equalled the nuclear capability of the USA.
The threat of Mutual Assured Destruction (MAD) created fear. This theory assumed that each superpower had enough nuclear weaponry to destroy the other. If one superpower attempted a first strike on the other, they themselves would also be destroyed. However, the MAD theory implied that both would be deterred from doing so.
The Cuban Missile Crisis (1962) emphasised to both sides the risk of not cooperating with each other. It was clear how easily nuclear war could have developed.
There had been confusion in communication between the USA and USSR during the crisis. This led to the creation of a hotline between Washington and Moscow. This would allow direct communication between both administrations.
During the crisis, tension had been intensified by continued US nuclear tests. These had made America appear more aggressive. The desire to reduce that pressure resulted in the 1963 Test Ban Treaty. This forbade tests in the atmosphere and was signed by the USA, the USSR and Britain.