When the new American ambassador to Japan, reportedly to be John Roos of California, steps off the airplane in Tokyo, the first thing he might want to do is to reassure the Japanese people in public and the Japanese government in private that the U.S. intends to fulfill its commitments to Japan's security, particularly with its nuclear umbrella.

That obligation, rooted in a security treaty and U.S. policy for decades, calls for the U.S. to retaliate against an aggressor mounting a nuclear attack on Japan. That pledge has increasingly come into question as Japanese political leaders, defense analysts and news commentators have wondered whether the U.S., and notably President Obama, can be trusted with the nuclear defense of Japan.

Moreover, mutterings of Japanese distrust of America's extended deterrence, as the nuclear umbrella is known, have coursed through a skeptical underground discussion. A Japanese scholar said: "There are a lot of Gaullists in disguise in Japan."

A Japanese diplomat, Yukio Satoh, one-time ambassador to the United Nations, wrote recently that "extended nuclear deterrence will continue to be Japan's only strategic option to neutralize potential or conceivable nuclear and other strategic threats." Thus, Satoh wrote, "the Japanese have been more concerned about the credibility of the American commitment."

That anxiety has reinvigorated a debate about whether Japan should acquire a nuclear deterrent of its own and reduce its reliance on the U.S. Japan has the technology, finances, industrial capacity and skilled personnel to build a nuclear force, although it would be costly and take many years.

The consequences of that decision would be earthshaking. It would likely cause opponents to riot in the streets and could bring down a government. South Korea, having sought at least once to acquire nuclear weapons, would almost certainly do so. Any hope of dissuading North Korea from building a nuclear force would disappear. China would redouble its nuclear programs.

And for the only nation ever to experience atomic bombing to acquire nuclear arms would surely shatter the already fragile international nuclear non-proliferation regime.

The main reason Japan has not acquired nuclear arms so far has been a lack of political will. After the atomic bombings of Hiroshima and Nagasaki in 1945, the Japanese experienced a deep-seated nuclear allergy. That and the threat from the Soviet Union during the Cold War kept Japan huddled under the U.S. nuclear umbrella.

Today, Japanese fear North Korea, which is developing nuclear weapons and has test fired missiles over Japan. Longer run, Japan casts wary eyes on China and Russia.

In addition, Japanese noted that Obama campaigned for election on a pledge to reduce nuclear arms. Once in office, he has called for their elimination, particularly during a speech in Prague several weeks ago. That has led many Japanese to question whether Obama can proceed on nuclear arms reduction and maintain the U.S. commitment to Japan's nuclear defense.

The former U.S. ambassador in Tokyo, Thomas Schieffer, appointed by President George W. Bush, said in a farewell appearance in January that the late French president, Charles de Gaulle, made "the argument that I sometimes hear reflected in arguments here in Japan, and that basically was that the United States couldn't be counted on to deter the Soviet Union when it came to nuclear weapons."

"I think that was absolutely wrong," Schieffer said. Japanese sometimes argue that "the U.S. will not risk New York for Tokyo," he said. "Well, we have. We've been doing that for 60 years, and that's not going to change."

The appointment of the new ambassador, John Roos, had not been announced as this was written. An attorney and Obama fundraiser from California, he must be confirmed by the Senate before going to Tokyo. While waiting, he might want to read up on the nuclear issue that will confront him when he lands there.

Richard Halloran, formerly with The New York Times as a correspondent in Asia and in Washington, is a writer in Honolulu.