WASHINGTON -- Despite its aversion to nuclear weapons, Japan allowed more American nuclear weapons on its territory during the 1950s and '60s than officials of either country have publicly acknowledged, according to declassified U.S. government documents.   Nuclear weapons for U.S. planes, submarines and surface ships were located on two Japanese islands -- Iwo Jima and Chichi Jima -- before the United States returned the islands to full Japanese control in 1968, according to the documents, which cite the types of weapons at various locations but not their numbers.

  After the United States ended its occupation of Japan in 1951 and the World War II enemies signed a security treaty, it was Japan's official policy not to permit nuclear weapons on its territory. Washington took the view that this prohibition did not extend to islands which remained under U.S. jurisdiction after 1951, according to a Clinton administration official who spoke on condition of anonymity.

  In 1997, secret U.S. government documents were declassified and the public learned that the island of Okinawa had been home to American nuclear weapons before it was returned to Japanese control in 1972. But the role of Iwo Jima and Chichi Jima was first disclosed in a report to be published Monday in the Bulletin of the Atomic Scientists by nuclear historians Robert S. Norris, William Arkin and William Burr.

  ''Fabled as a 'non-nuclear nation,' Japan is beginning to look very different, given what we now know,'' the authors wrote. ''Japan may have had its principles, but the Pentagon had its nuclear war plans and it pushed the envelope as far as it could.''

  Pentagon spokesman P.J. Crowley said Friday that the government documents on which the Bulletin based its article are authentic, but he said the U.S. government is sticking to its policy of neither confirming nor denying the presence of nuclear weapons at any location, either now or in the past.  ''It is in our interest to continue to maintain a necessary level of ambiguity about these systems,'' Crowley said.  In the early 1950s, U.S. officials believed they needed to have nuclear weapons, or their non-nuclear components, dispersed in Europe and in the Pacific in case war broke out with the Soviet Union. They were deployed by the thousands in such places as South Korea, Guam, the Philippines and Taiwan. As defense strategies evolved in the Cold War, the Pentagon consolidated its arsenal. Today, the only U.S. nuclear weapons deployed outside the United States are bombs for aircraft stationed in several European NATO countries.

The new disclosures about U.S. nuclear weapons in Japan are not likely to affect U.S.-Japan relations today, but they fill a gap in the historical record of Japan's role in supporting U.S. nuclear war plans.

There is no evidence that the U.S. government ever obtained permission from Japan to store complete nuclear weapons on the main islands. Yet a declassified appendix to a secret U.S. Far East Command report, dated Nov. 1, 1956, indicates that 13 separate locations in Japan -- including sites on the main islands -- had nuclear weapons or components or were earmarked to receive weapons in the event of impending war.  These included Misawa, Itazuki, Atsugi, Iwakuni, Johnson and Komaki air bases on the mainland, although it is not clear from the available records whether complete nuclear weapons ever were placed at these sites.

The roles of Chichi Jima and Iwo Jima as nuclear storage sites are clearer.  A top secret June 1957 memorandum for Adm. Arthur Radford, then chairman of the Joint Chiefs of Staff, discussed the dispersal of nuclear weapons in the Bonin and Volcano islands. The Bonins are a group of Japanese islands, about 500 miles southeast of the mainland, of which only Chichi Jima is inhabited. In the three-island Volcano group, 120 miles from Chichi Jima, only Iwo Jima played a military role.

  ''On 6 February 1956 the chief of naval operations stated that one weapon with core was placed in storage on Chichi Jima,'' the Radford memo said.  The records do not state how many bombs were placed on Chichi Jima. A Pentagon history of the deployment of U.S. nuclear weapons, declassified earlier this year, showed that the first bombs on Chichi Jima were withdrawn after only three months, coinciding with the introduction of the Navy's Regulus nuclear missiles, which were fired from submarines. These missiles were kept on Chichi Jima until 1964. The last nuclear weapons on the island, W30 warheads for Navy surface-to-air Talos missiles, were withdrawn in December 1965.

  Because of Chichi Jima's nuclear role, the U.S. government resisted Japan's push to repopulate the island, which had been evacuated during World War II. Mansfield Sprague, an assistant secretary of defense in the Eisenhower administration, told the ''CNN & Time'' news program that U.S. officials felt that allowing the Japanese to return was too big of a security risk.

  ''Well, if they'd been able to come back to the islands, some of them might have been spies,'' Sprague said in a ''CNN & Time'' interview to be broadcast Sunday.

 Also in February 1956, ''non-nuclear bombs'' -- components without the nuclear core -- were placed on Iwo Jima and kept there until June 1966, according to the Pentagon's ''History of the Custody and Deployment of Nuclear Weapons.'' Compete nuclear bombs were on Iwo Jima from September 1956 to December 1959.  Declassified records of the U.S. Far East Command show that Detachment One of the Air Force's 7th Tactical Depot Squadron had a nuclear storage site at Central Air Base on Iwo Jima. Norris and his co-authors interviewed a former Air Force officer assigned there who told them the island served as a ''recovery facility.'' In the event of nuclear war, bombers which had released their weapons over the Soviet Union or China were to fly to Iwo Jima to be refueled and reloaded with bombs for a second attack.

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