Nuclear warfare
Scientists in the United States from the Manhattan Project had warned that, in time, the Soviet Union would certainly develop nuclear capabilities of its own. Nevertheless, the effect upon military thinking and planning in the United States was dramatic, primarily because American military strategists had not anticipated the Soviets would "catch up" so soon. However, at this time, they had not discovered that the Soviets had conducted significant nuclear espionage of the project from spies at Los Alamos National Laboratory, the most significant of which was done by the theoretical physicist Klaus Fuchs. The first Soviet bomb was more or less a deliberate copy of the Fat Man plutonium device. In the same year the first US-Soviet nuclear war plan was penned in the US with Operation Dropshot.
With the monopoly over nuclear technology broken, worldwide nuclear proliferation accelerated. The United Kingdom tested its first independent atomic bomb in 1952, followed by France developing its first atomic bomb in 1960 and then China developing its first atomic bomb in 1964. While much smaller than the arsenals of the United States and the Soviet Union, Western Europe's nuclear reserves were nevertheless a significant factor in strategic planning during the Cold War. A top-secret White paper, compiled by the Royal Air Force and produced for the British Government in 1959, estimated that British V bombers carrying nuclear weapons were capable of destroying key cities and military targets in the Soviet Union, with an estimated 16 million deaths in the Soviet Union (half of whom were estimated to be killed on impact and the rest fatally injured) before bomber aircraft from the U.S. Strategic Air Command reached their targets.

1950s: Although the Soviet Union had nuclear weapon capabilities at the beginning of the Cold War, the United States still had an advantage in terms of bombers and weapons. In any exchange of hostilities, the United States would have been capable of bombing the Soviet Union, whereas the Soviet Union would have more difficulty carrying out the reverse mission.
The widespread introduction of jet-powered interceptor aircraft upset this imbalance somewhat by reducing the effectiveness of the American bomber fleet. In 1949 Curtis LeMay was placed in command of the Strategic Air Command and instituted a program to update the bomber fleet to one that was all-jet. During the early 1950s the B-47 Stratojet and B-52 Stratofortress were introduced, providing the ability to bomb the Soviet Union more easily. Before the development of a capable strategic missile force in the Soviet Union, much of the war-fighting doctrine held by western nations revolved around using a large number of smaller nuclear weapons in a tactical role. It is debatable whether such use could be considered "limited" however because it was believed that the United States would use its own strategic weapons (mainly bombers at the time) should the Soviet Union deploy any kind of nuclear weapon against civilian targets. Douglas MacArthur, an American general, was fired by President Harry Truman, partially because he persistently requested permission to use his own discretion in deciding whether to utilize atomic weapons on the People's Republic of China in 1951 during the Korean War. Mao Zedong, China's communist leader, gave the impression that he would welcome a nuclear war with the capitalists because it would annihilate what he viewed as their imperialist system.

Let us imagine how many people would die if war breaks out. There are 2.7 billion people in the world, and a third could be lost. If it is a little higher it could be half ... I say that if the worst came to the worst and one-half dies, there will still be one-half left, but imperialism would be razed to the ground and the whole world would become socialist. After a few years there would be 2.7 billion people again.

The concept of a "Fortress North America" emerged during the Second World War and persisted into the Cold War to refer to the option of defending Canada and the United States against their enemies if the rest of the world were lost to them. This option was rejected with the formation of NATO and the decision to permanently station troops in Europe.
In the summer of 1951, Project Vista started, in which project analysts such as Robert F. Christy looked at how to defend Western Europe from a Soviet invasion. The emerging development of tactical nuclear weapons was looked upon as a means to give Western forces a qualitative advantage over the Soviet numerical supremacy in conventional weapons.
Several scares about the increasing ability of the Soviet Union's strategic bomber forces surfaced during the 1950s. The defensive response by the United States was to deploy a fairly strong "layered defense" consisting of interceptor aircraft and anti-aircraft missiles, like the Nike, and guns, like the M51 Skysweeper, near larger cities. However, this was a small response compared to the construction of a huge fleet of nuclear bombers. The principal nuclear strategy was to massively penetrate the Soviet Union. Because such a large area could not be defended against this overwhelming attack in any credible way, the Soviet Union would lose any exchange.
This logic became ingrained in American nuclear doctrine and persisted for much of the duration of the Cold War. As long as the strategic American nuclear forces could overwhelm their Soviet counterparts, a Soviet pre-emptive strike could be averted. Moreover, the Soviet Union could not afford to build any reasonable counterforce, as the economic output of the United States was far larger than that of the Soviets, and they would be unable to achieve "nuclear parity".
Soviet nuclear doctrine, however, did not match American nuclear doctrine. Soviet military planners assumed they could win a nuclear war. Therefore, they expected a large-scale nuclear exchange, followed by a "conventional war" which itself would involve heavy use of tactical nuclear weapons. American doctrine rather assumed that Soviet doctrine was similar, with the mutual in mutually assured destruction necessarily requiring that the other side see things in much the same way, rather than believing—as the Soviets did—that they could fight a large-scale, "combined nuclear and conventional" war.
In accordance with their doctrine, the Soviet Union conducted large-scale military exercises to explore the possibility of defensive and offensive warfare during a nuclear war. The exercise, under the code name of "Snowball", involved the detonation of a nuclear bomb about twice as powerful as that which fell on Nagasaki and an army of approximately 45,000 soldiers on maneuvers through the hypocenter immediately after the blast. The exercise was conducted on September 14, 1954, under command of Marshal Georgy Zhukov to the north of Totskoye village in Orenburg Oblast, Russia.
A revolution in nuclear strategic thought occurred with the introduction of the intercontinental ballistic missile (ICBM), which the Soviet Union first successfully tested in August 1957. In order to deliver a warhead to a target, a missile was much faster and more cost-effective than a bomber, and enjoyed a higher survivability due to the enormous difficulty of interception of the ICBMs (due to their high altitude and extreme speed). The Soviet Union could now afford to achieve nuclear parity with the United States in raw numbers, although for a time, they appeared to have chosen not to.
Photos of Soviet missile sites set off a wave of panic in the U.S. military, something the launch of Sputnik would do for the American public a few months later. Politicians, notably then-U.S. Senator John F. Kennedy suggested that a "missile gap" existed between the Soviet Union and the United States. The US military gave missile development programs the highest national priority, and several spy aircraft and reconnaissance satellites were designed and deployed to observe Soviet progress.
Early ICBMs and bombers were relatively inaccurate, which led to the concept of countervalue strikes — attacks directly on the enemy population, which would theoretically lead to a collapse of the enemy's will to fight. During the Cold War, the Soviet Union invested in extensive protected civilian infrastructure, such as large "nuclear-proof" bunkers and non-perishable food stores. By comparison, smaller scale civil defense programs were instituted in the United States starting in the 1950s, where schools and other public buildings had basements stocked with non-perishable food supplies, canned water, first aid, and dosimeter and Geiger counter radiation-measuring devices. Many of the locations were given "fallout shelter" designation signs. CONELRAD radio information systems were adopted, whereby the commercial radio sector (later supplemented by the National Emergency Alarm Repeaters) would broadcast on two AM radio frequencies in the event of a Civil Defense (CD) emergency. These two frequencies, 640 and 1240 kHz, were marked with small CD triangles on the tuning dial of radios of the period, as can still be seen on 1950s-vintage radios on online auction sites and museums. A few backyard fallout shelters were built by private individuals.
Henry Kissinger's view on tactical nuclear war in his controversial 1957 book Nuclear Weapons and Foreign Policy was that any nuclear weapon exploded in air burst mode that was below 500 kilotons in yield and thus averting serious fallout, may be more decisive and less costly in human lives than a protracted conventional war.
A list of targets made by the United States was released sometime during December 2015 by the U.S. National Archives and Records Administration. The language used to describe targets is "designated ground zeros". The list was released after a request was made during 2006 by William Burr who belongs to a research group at George Washington University, and belongs to a previously top-secret 800-page document. The list is entitled "Atomic Weapons Requirements Study for 1959" and was produced by U.S. Strategic Air Command during the year 1956.