Project Islero was an attempted Spanish nuclear program. Named after Islero, the bull which felled the famous bullfighter Manolete, the program was created by Generals Agustín Muñoz Grandes and Guillermo Velarde [es] in 1963. Although Spain possessed the second largest uranium deposits in the world at the time, it was not until the Palomares Incident of 1966 that Spain would focus on plutonium-239 implosion-type designs. Yet, in 1966, Franco paused the military research, shifting efforts to nuclear reactor construction and plutonium production. However, the program was resumed in 1971, with help from Charles de Gaulle's France to refine the material and fund the nuclear facilities.

Lasting from the middle stages [es] of Francisco Franco's rule into the beginning of the democratic transition, the project was prematurely cancelled due to American pressures in 1981, although it only formally ended in 1987 under Felipe González. Although the project never developed a nuclear weapon, the country possessed the capabilities to both design and manufacture the necessary components, with the Spanish Foreign Minister, José María de Areilza, declaring in 1976 that Spain would be able to manufacture the bomb "in seven or eight years if we set our minds to it."

In September 1948, by means of a secret decree, Francisco Franco created the Junta de Investigaciones Atómicas (JIA), or Board for Nuclear Research. Constituted on October 8, the board was formed by José María Otero de Navascués [es] (general director and president until 1974), Manuel Lora-Tamayo, Armando Durán Miranda [es] and José Ramón Sobredo y Rioboo [es]. In 1951, the secret phase was declared over, and the JIA was renamed the Junta de Energía Nuclear (JEN), or Board of Nuclear Energy, inaugurated in the University City of Madrid under the presidency of General Juan Vigón and with Otero de Navascués as general director. The purpose of the JEN was to work "as a research center, as an advisory body to the Government, as an institute in charge of safety and protection against the danger of ionizing radiation, and as a driving force for industrial development in the field of nuclear energy applications".

On 1 April 1939, the United States lifted the embargoes placed on Spain after the Spanish Civil War, providing limited recognition to the Spanish State, and the embassy to Spain, previously in Barcelona, now headed by a Chargé d'Affaires ad interim, was moved back to Madrid on the 13th. Yet with Spain acting as all but a member of the Axis powers, there was much skepticism among the Western world as to whether or not they should be allowed to join the organs of the new international order such as the United Nations. Public opinion of the Spanish State was low. At the San Francisco Conference, Spain was barred, while prominent Spanish Republican leaders were in attendance, exerting a notorious influence on several delegations, extended to the conditions of entry into the United Nations.

As well, at the Potsdam Conference, the question of how to proceed with post-war Spain was one of the first to be dealt with. In this sense, Stalin was, in a certain way, seeking revenge against the Francoist State, due in part to the fact that the State had sent the Blue Division (volunteers fighting with the German armed forces) to the Soviet Union during World War II, with a joint statement from the Big Three reading:

The Three Governments feel bound however to make it clear that they for their part would not favour any application for membership put forward by the present Spanish Government, which, having been founded with the support of the Axis Powers, does not, in view of its origins, its nature, its record and its close association with the aggressor States, possess the qualifications necessary to justify such membership.

Yet as the Cold War began, and with Spanish agents, US military officials, and US businessmen lobbying for the opening of relations, popular opinion shifted. The appointment of a US ambassador to Madrid was announced on 27 December 1950. In July 1951, negotiations started for an alliance which would eventually become the Pact of Madrid, and thus in 1955, amidst warming of relations between Spain and the United States as well as Spanish ascension to the UN, Francisco Franco and President Dwight D. Eisenhower signed an agreement of nuclear cooperation as part of the Atoms for Peace initiative, opening up nuclear research to civilians and countries that had not previously possessed nuclear technology. Eisenhower, a proponent of non-proliferation, sought to stop the spread of military use of nuclear weapons. Although the nations that already had atomic weapons kept their weapons and grew their supplies, the program was designed to prevent other countries from developing similar weapons. The program also created regulations for the use of nuclear power, aiming to shape the use of nuclear power into a solely positive means.

This would not necessarily work towards the desired results; in fact, it would be this very program which enabled General Franco, accompanied by the Minister of the Presidency of the Government, Luis Carrero Blanco, to inaugurate the Juan Vigón National Nuclear Energy Center at its facilities in the University City of Madrid on December 27, 1958. Together with the 1959 Stabilization Plan and the subsequent Spanish miracle, Spain was able to begin shifting away from the previous autarky, allowing for the beginning of a fledgling civilian nuclear sector which would lay the foundations for Project Islero.