The purpose of this paper is to clarify empirically the political and diplomatic processes centering on nuclear submarines’ visits to Japan in 1964. When the Security Treaty with the United States was revised in 1960, Japan legally stipulated that bringing nuclear weapons into its territory should be a matter for prior consultation. However, during the negotiations to revise the treaty, it did not place explicitly on the agenda the issue of port calls by nuclear-armed ships.

On the other hand, the U.S. was rapidly advancing the development of nuclear weapons to be mounted on submarines. After the revision of the treaty, it successfully launched a Polaris missile from an underwater nuclear submarine, and proceeded with the development of Subrocs to be mounted on submarines. Under these circumstances, in June 1961 and January 1963, it requested Japan to accept visits by its nuclear submarines.

The development of nuclear weapons technology and the existence of public information on it are factors that are essentially outside of alliance politics. And the former was originally intended by the U.S. to maintain its superiority over the Soviet Union and increase its credibility in the eyes of the allies. The development of Polaris missiles and Subrocs, however, put the Japanese government in a difficult position in domestic politics. Opposition legislators were able to grasp the development status of new nuclear weapons from information released by the U.S. and to take up the issue in the Diet. The Ministry of Foreign Affairs thus had to consider how to handle, under the prior consultation system, cases of visits by submarines equipped with Polaris missiles and Subrocs.

At the time MOFA, on the basis of pre-existing official documents and government statements that had been made in the Diet, offered legal and policy interpretations of such cases. However, there were various limitations in applying to sea-based nuclear weapons past policies that presupposed those that were land-based.

In the end, while the government could not officially allow visits by nuclear-armed ships to Japanese ports, it also fell into a situation where it could not come to an explicit agreement with the United States on how to handle such visits under the Security Treaty. In this way, the development of new nuclear weapons to increase America’s credibility in the eyes of its allies had rather the political consequence of creating, for Japan, an alliance management dilemma. In 1964, in the absence of any resolution of the dilemma, Japan made a highly political decision to allow nuclear submarines to visit its ports.