Mark 3 – “Fat Man” plutonium implosion weapon (used against Nagasaki), effectively the same as the “Gadget” device used in the Trinity (nuclear test) with minor design differences. (21 kilotons, 1945–1950)
Mark 4 – Post-war “Fat Man” redesign. Bomb designed with weapon characteristics as the foremost criteria. (1949–1953)
Mark 5 – Significantly smaller high efficiency nuclear bomb. (1–120 kilotons, 1952–1963)
Mark 6 – Improved version of Mk-4. (8–160 kilotons, 1951–1962)
Mark 7 – Multi-purpose tactical bomb. (8–61 kilotons, 1952–1967)
Mark 8 – Gun-assembly, HEU weapon designed for penetrating hardened targets. (25–30 kilotons, 1951–1957)
At the end of World War II, the defeated Axis nations and many of the victorious allied countries were devastated, their economies and industrial capabilities unable to support even a peacetime society. In terms of war-fighting capability, the USSR possessed a large and well-armed ground force that could easily have overrun Western Europe, or possibly even China. The USSR was unable to sustain such a force, however, because of catastrophic conditions in the homeland, nor did it have a long-range air or naval capability to support an aggressive national strategy.
The prevalent strategic view in the Truman phase of the air “atomic age” (1945-53) was that long-range bombers carrying nuclear weapons against enemy cities or military forces could defeat any nation or force hostile to the United States and its interests. In this environment, the US Air Force was established as a separate service, and the Army and Navy were reduced essentially to “token” forces. These small ground and naval services would be required in future conflicts primarily to provide certain occupation and logistic forces to support the primary weapon: the long-range bomber.
On June 25, 1950, ground and air forces of Communist North Korea crossed the border into South Korea in an all-out assault to gain control over the entire Korean peninsula. The perceived US strategy was articulated by one Air Force Officer who, when told that US ground troops were to be committed to the war, is said to have remarked: “The old man [General MacArthur] must be off his rocker. When the Fifth Air Force gets to work on them, there will not be a North Korean left in North Korea.”
Only after three full years of conventional warfare, involving mostly US air, ground, and naval weapons of World War II vintage, was the Korean War finally brought to a conclusion. US forces had achieved their goal of maintaining the independence of South Korea without the employment of “tactical” or “strategic” nuclear weapons. Both uses were considered: tactical, in the sense of direct support to ground operations (against troop concentrations, bridges, and so forth); and strategic, against mainly factories and assembly areas in North Korea and Manchuria. President Truman apparently gave consideration to the use of nuclear weapons against the Soviet Union in this period. A journal kept in his own handwriting has an entry dated January 27, 1952, contemplating a threat of an “all-out war” against the Soviet Union as well as China: “It means that Moscow, St. Petersburg [Leningrad], Vladivostok, Peking, Shanghai, Port Arthur, Dairen, Odessa, Stalingrad, and every manufacturing plant in China and the Soviet Union will be eliminated.”
After the Korean War ended in stalemate in mid-1953 the war in Indochina, between French forces and the Communist Vietminh, continued in a deadly struggle. A large French force was surrounded by the Vietminh at Dienbienphu, with the end of the Korean War freeing guns, munitions, and technical advisors for the fight against the French. By the spring of 1954 the situation at Dienbienphu was critical and the French asked for American air strikes, first conventional and then nuclear (under the code name Vulture). B-29s flying from the Philippines or carrier aircraft could have carried out the strikes. Although President Eisenhower, the Secretary of State, and four of the five members of the U. S. Joint Chiefs of Staff favored direct intervention, British opposition and French reluctance to accept American direction led to an end of Operation Vulture. Dienbienphu fell to the Communists on May 7, 1954. It was a severe military defeat for the French and-more importantly-led to a political end of the conflict.
At the time the same nuclear weapons would have been used against tactical targets (Korea and Indochina) and strategic targets (the Soviet Union and China). Although American scientists during this period were developing relatively small nuclear weapons, at the time of Truman’s journal entry the atomic bombs available were few in number and large in size. The Mk 3 and Mk 4 weapons then in the inventory were more than ten feet long, five feet in diameter, and weighed over five tons. Under normal circumstances, the bombs were not assembled; to put them together required a crew of trained technicians and almost a day.
As the Cold War increased in intensity, President Truman ordered an increase in nuclear weapons production. The American arsenal grew from perhaps seven bombs-or, more accurately, the components for seven- in mid-1947, to about 25 a year later, and to some 50 in mid-1949. Indications are that by mid-1950 production had provided an arsenal of at least 300, with approximately another hundred being added each year during the Korean War, and even more after that-possibly even totalling as many as 2,000 by mid-1955. Under the Truman Administration there occurred not only an increase in numbers, but a diversification of types: the Mk 5, which weighed only 3,000 pounds, had retractable fins, and was the first nuclear weapon that could be carried externally on an aircraft; the Mk 6 (8,500 pounds), which was the first to be mass produced; the small, 1,600-pound Mk 7 that could be used as a missile warhead as well as a bomb; and the Mk 8 (3,300 pounds), which was intended to penetrate hardened structures. All four weapons were produced from 1951-52 onward. Thus, after an initial period of possessing virtually no useable nuclear capability, by the early 1950s the United States was becoming a true nuclear power.
Subsequently, the Eisenhower-Dulles Administration (1953-61) enunciated the strategy that became known as “massive retaliation.” According to this doctrine, aggression against the United States or its allies would be deterred with the threat of massive retaliatory nuclear strikes; if deterrence should fail, the US would prevail against the Soviet Union in a general war. The doctrine, however, also called for conventional forces to deter or contain localized aggression without resorting to nuclear weapons. While the mix and balance of conventional forces and nuclear weapons were not specified in the major policy documents, the Army and Navy both sought to modernize and maintain large conventional forces. This, in turn, further reinforced proponents of nuclear weapons as a means of controlling defense spending through the use of a relatively small Air Force and Navy nuclear attack forces. In this period, the ability of the Soviet Union to attack the United States with nuclear weapons consisted of a few long-range Soviet nuclear bombers that would have to survive both the long flight to the United States (no bases in the Western hemisphere being available) and the expanding US warning and air defense system. Also, in the 1950s an active US civil defense program was in existence.