Japan - the only country to suffer nuclear attack - is committed to pacifism
In 1965 Japan asked the US to be ready to launch a nuclear attack on China if war broke out between the Asian rivals, documents from the time indicate.
The documents, declassified by Japan's foreign ministry, summarise talks held during a visit to Washington by Japan's then prime minister, Eisaku Sato.
Mr Sato won the Nobel peace prize in 1974 for his rejection of nuclear weapons.
Japan is committed to pacifism under the terms of its post-war constitution.
It is the only country in the world to have suffered a nuclear attack.
Mr Sato projected a strong public stance against nuclear weapons, and formulated Japan's three-point non-nuclear policy which pledged that the country would not produce, possess or allow nuclear weapons on its territory, and which is still adhered to today.
But the newly declassified documents show that three years earlier and behind closed doors, Mr Sato - Japan's longest serving prime minister - adopted a rather different stance towards nuclear weapons.
His talks with then US Secretary of Defense Robert McNamara took place on 13 January 1965, against the backdrop of China's first successful test of an atomic bomb some months beforehand.
Mr Sato won a Nobel Peace Prize for his anti-nuclear stance
Neither Japan nor the US had diplomatic relations with China, and in some circles it was viewed as a serious threat.
According to a summary of their talks, written mostly in Japanese, after mentioning the Chinese test Mr McNamara said a key issue would be whether Japan moved to develop its own offensive nuclear capability.
In response, Mr Sato told Mr McNamara that while Japan had the technical capability to build atomic weapons, it had no intention of doing so.
But he added that it would "of course be a different matter in the event of a war", adding "we expect the United States to retaliate immediately using nuclear [weapons]".
He also said that he would allow the US to use Japanese waters - though not Japanese land - to launch such an attack.
A day before the discussion, Mr Sato had met US President Lyndon Johnson and asked for a guarantee of protection for his country under the Japan-US security treaty, and received an assurance of Washington's continued commitment to the pact.
Responding to the revelations, Japan's current Chief Cabinet Secretary Takeo Kawamura defended Mr Sato's moves on the grounds that China had just tested its own nuclear bomb at the time.
He said Mr Sato's three-point anti-nuclear policy was "determined and steadfast", AFP news agency reported.
The Chinese foreign ministry issued no immediate comment on the declassified documents.