Japan and the U.S. signed a secret nuclear pact when they revised their security treaty in 1960, a Japanese Foreign Ministry probe has revealed according to the Asahi Shimbun last Saturday. The existence of such a pact had been the subject of conjecture for some time, and as the new ruling Democratic Party continues to dig into past dealings between the U.S. and Japan, this could pose new threats to the already tense relationship between Washington and Tokyo.

The relationship was barely patched up following conflicts involving the relocation of the Futenma U.S. Marine Base in Okinawa and Japan's refusal to continue its mission on the Indian Ocean to refuel multinational ships involved in operations in Afghanistan. 

Protesters aboard small vessels encircle the aircraft carrier USS John C. Stennis in an attempt to prevent it from docking in Sasebo port, Japan, on Aug. 21, 2004. /AP

◆ Nuclear Weapons on Japanese Soil

When the Japan-U.S. Security Treaty was revised in 1960, one of the key items on the agenda was a pledge by Washington to consult with Tokyo prior to any deployment of mid-to-long-range missiles or nuclear weapons on Japanese soil. But there were suspicions that no such consultations would be needed when U.S. bombers carrying nuclear weapons flew over Japanese airspace or if nuclear-equipped American ships docked in Japanese ports.

The Liberal Democratic Party governments of the last 49 years had consistently denied such speculation despite the discovery of declassified U.S. diplomatic documents that showed such a secret pact existed. This tacit authorization of nuclear weapons on Japanese soil goes against the three anti-nuclear principles Tokyo announced in 1967.

They forbid the possession, production and introduction of nuclear weapons in Japan and served as the grounds for Japan's pressure on North Korea to scrap its nuclear weapons program.

The Democratic Party, which assumed power on Sept. 16, began sifting through 3,500 diplomatic documents aiming to resolve suspicions over any secret pacts.

◆ The Documents

Japanese officials have yet to reveal the documents that have been discovered. Some time next week, Foreign Minister Katsuya Okada and his staff plan to put together a team of experts to authenticate the documents and then reveal them around January. But the Japanese media have already reported that the revelations are highly likely to be about the existence of the secret nuclear pact.

The issue dates back decades and does not have a huge impact on Japan's modern relations with the U.S., and the U.S. government removed tactical nuclear weapons from Northeast Asia including the Korean Peninsula in 1991. But the reason the matter is stirring up tremendous controversy in Japan is because the Tokyo-Washington alliance has been shaky since the launch of the Hatoyama administration.

Starting with the controversial issue of relocating the Futenma U.S. Marine base, many Japanese believe existing rules to protect U.S. soldiers in Japan are unfair and must be redressed. Next year marks the 50th anniversary of the security treaty between the U.S. and Japan.

In the Nov. 13 meeting between Japanese Prime Minister Yukio Hatoyama and U.S. President Barack Obama, the two leaders agreed to begin talks over a new security treaty.