Named after Islero, the bull which fell the famous bullfighter Manolete, the program was created by Generals Agustín Muñoz Grandes and Guillermo Velarde [es] in 1963.

[1]: 497–498 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,[2] although it only formally ended in 1987 under Felipe González.

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".

[11] 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.

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,[11] 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.

This would not necessarily work towards the desired results;[a] 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.

[24] The Spanish state would undergo secret research on the debris zones of the incident, with technicians led by Velarde finding remains of the bomb and the detonators in the area.

Yet later that year, Franco would hold a meeting with Velarde in which he ordered to postpone indefinitely the physical, but not theoretical, development of the project due to fears that it would be impossible to keep a secret, and with the recent creation of the International Atomic Energy Agency (IAEA), publicity would lead to increased economic sanctions on Spain.

He did, however, allow the research to go ahead, albeit detached from the Armed Forces; as well Franco pledged not to sign an international agreement being negotiated at the time to prohibit the manufacture of nuclear weapons.

Obtaining enough plutonium to build the bomb (6 kilos), in a country whose subsoil contained the second largest natural uranium reserves in Europe, was no longer considered a dream with the help of the French, who themselves did not allow the IAEA to inspect their plants.

Both General Charles De Gaulle and Georges Pompidou, his successor as President of France, were in favor of the project from the outset; the presence of an allied nuclear power on the continent was seen as a means of increasing French independence from both the United States and NATO.

[3] As well, according to some confidential reports declassified by the US Military Intelligence Service, Spain was storing plutonium to manufacture a nuclear bomb, diverting it from IAEA controls; something completely out of line for an American ally, an action on par with a rogue state.

The CIA informed U.S. Secretary of State, Henry Kissinger that Project Islero would soon bear fruit, and with tensions between Spain and America on the rise due to the 1973 Oil Crisis and Yom Kippur War, a meeting was soon called with Carrero Blanco.

Kissinger, although failing to acquire Spanish adhesion to the NPT, did convey a clear message: the confirmation of Franco's desire for military nuclear devices made a "tight control" necessary over these activities.

The minister had spoken to Franco to convince him to put an end to it, arguing that the Americans would eventually get wind of the project and that this would cause Spain an endless series of problems.

Franco put an end to the heated discussions by ordering a halt to the research and forbidding the military project to be set in motion, informing Velarde that "Spain could not support a new international blockade unleashed by the United States, and the benefits of having a small arsenal did not outweigh the damage".

Suárez, who advocated a policy of neutrality and friendship towards the Arab world to prevent problems in the Canary Islands, Ceuta, and Melilla, supported the idea of a Spanish atomic bomb as a final deterrent against any armed aggression.

[3][4] While internally the project was well underway, externally, American pressures continued to mount; during his four-year rule, President Jimmy Carter had launched a campaign against the states that had not signed the NPT.