Newly declassified Japanese diplomatic documents show that the late Prime Minister Eisaku Sato expressed his expectation to then U.S. Defense Secretary Robert McNamara that the United States would immediately retaliate against China with nuclear weapons if war occurred between China and Japan.

Sato spoke with Mr. McNamara on Jan. 13, 1965, in Washington after meeting with President Lyndon B. Johnson. He also suggested that the U.S. could immediately use sea-based nuclear weapons.

On the surface, Sato’s statement appears to contradict Japan’s nonnuclear policy. But it should not be used as an excuse to deviate from Japan’s nonnuclear weapons policy.

After China carried out its first test of a nuclear explosion in October 1964, Sato apparently sought a guarantee that the U.S. would protect Japan under its nuclear umbrella. A separate summary of Sato’s meeting with Johnson shows that the American president replied positively to his request: “You have my assurance.”

But attention should be paid to another area of Sato’s discussion with Mr. McNamara, in which he stated that Japan had absolutely no interest in possessing nuclear weapons. He made it clear that although Japan had technical capabilities to make nuclear weapons, Japan would not follow the line of French President Charles de Gaulle, who pushed for the development of France’s own nuclear arsenal.

In the Diet in December 1967, Sato announced his Three Nonnuclear Principles: nonproduction, nonpossession and nonintroduction of nuclear weapons, and in 1974 received the Nobel Peace Prize for establishing this nonnuclear policy. It is important to remember that the government adopted the policy by taking into various factors including the diplomatic damage that “going nuclear” would cause.

If Japan decided to become a nuclear power it would have to withdraw from the Nuclear Nonproliferation Treaty, a basic pillar of the global order. Going nuclear would also cause mistrust among neighboring countries and upset the regional balance of power. Finally, Japan — the only nation to suffer atomic bombings — would lose its moral authority to persuade other nations to give up nuclear weapons.

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