The POD here is a nuclear war during the Cuban Missile Crisis, and small details are changed regarding the Crisis.
The Initial Crisis
Cuba, during early 1962, requested nuclear weapons from Khrushchev, and got ones able to hit the East Coast of much of the Americas. It openly acknowledged this fact, getting rage from America and its allies. America threatened to use nuclear weapons on Cuba, Venezuela and even innocent Mexico. Further nuclear weapons, much to the rage of NATO, were delivered later to Venezuela, which they could aim at much of Latin America and part of southern USA. The USA soon posts nuclear weapons in Chile, Argentina and Puerto Rico in a hurry. The stand off starts.
Decision to launch a Nuclear War
The missile crisis was getting unbearable for both sides, and Kennedy in particular was urged to trigger a limited nuclear war in Latin America. Kennedy clearly had considered this idea before, for he had once evacuated some Texans before. Meanwhile in the Soviet Union many party members, all patriotic, wanted to destroy the USA once and for all.
After an 80-day standoff, on 17th Sept 1962, President Kennedy ordered an evacuation in the South and prepared a battle plan for a nuclear advance into Latin America. He identified Cuba, Venezuela and to one extent, Mexico as major targets. On the 23rd, after the South people were evacuated and East Coast evacuations were half-finished, the First Nuclear War started.
The Nuclear War
Santa Clara was the first target struck by American troops in Floridan bases. Soon, a number of Cuban cities and bases were hit and quickly afterwards, missiles were launched towards Venezuela and Mexico. Mexico City and many others were struck in a matter of minutes. The first wave was over in seven minutes. The Americans, however, considered their missiles too precious to waste on military bases, and Cuba and Venezuela launched their own missiles. Chile and Argentina were soon hit. as was Eastern and Southern USA. Missiles hit New York, Philadelphia, Austin and more. A total of over 30 million died in the first two waves of attacks across the Americas. The limited exchange ends in 32 minutes, but a global war soon occurs.
The War Goes Global
The USSR was outraged by the strikes. Chavez and Castro had been killed before Soviet silos could alert them. It ordered a large deployment into Eastern Europe, North Korea, and China. Eastern European missiles were expected to be launched on the 27th. Iran was sold 20 nuclear missiles, and China 100 at premium cost. Iran was told to be anti-American and hit Middle Eastern and North African targets. On the 27th, hundreds of missiles were launched at Western Europe, Thailand, Turkey and Australia. Riyadh and Dubai, among others, were hit. Iran used all of its missiles, at Dubai, Abu Dhabi, Riyadh, Cairo, Jerusalem, Istanbul, Baghdad, Calcutta, Mumbai and so on, in the initial attack. China attacks the Pacific Area and Thailand. It also sends a large batch of 35 missiles to the US West Coast, which are mostly intercepted, although one hits San Francisco. Over 50 million die in the Soviet attacks, and more will in the counterattack next hour. Then, European states are busy launching back missiles. Moscow is hit. Khrushchev hides in a bunker, but the chaos in Moscow makes him feel ashamed and he commits suicide. Generals and party officials take control and direct an immediate attack on the East Coast. Washington, DC is hit out of a sudden and Kennedy, too, dies of the powerful impact of five nuclear missiles, in his bunker. The nuclear exchange's counterstrike kills another 50.3 million. With the war's death toll at about 130.3 million dead, including many leaders, the world starts to descend into chaos.
NATO Counterstrike and Soviet Second Strike
NATO prepared a counterattack drawn up by the surviving world leaders. Together on 4th October 1962, many remaining nuclear, biological and chemical weapons were launched soon, to the remains of the USSR and its allies. The generals have to launch theirs at the same time. Biological weapons strike surviving Japan, South Korea, North Korea and China. The Soviet Union now launches more missiles at the USA. In this assault, about 70.7 million die, totaling the toll at over 201 million, about 1 in 30. The deaths are considerably less than some doomsday scenarios, but the "last strike" that comes will be deadly.
Warsaw Pact initiated this strike, and NATO responded. In this strike, military bases and cities were targeted. Soon both sides are utterly destroyed with over 300 million dead, the death toll is now over 501 million, or 0.501 billion. Biological weapons, chemical weapons, untreated diseases, fallout and climatic changes kill about 300,000 instantly. Many countries in the world are destroyed, and the world descends into chaos.
The USA quickly plunged into chaos. The town areas of the South and Central survive, relatively speaking.