In a little noted change in U.S. nuclear policy, President Obama last week threatened to employ nuclear weapons against North Korea in retaliation for any nuclear attack on South Korea.

Rarely, if ever, has the U.S. disclosed when or under what circumstances or in which country it would use nuclear weapons. Instead, U.S. nuclear doctrine has been wrapped in generalities and ambiguity intended to deter a potential adversary from a nuclear attack by keeping it guessing. Day-to-day, that doctrine calls for never confirming or denying the presence of U.S. nuclear weapons anywhere.

Last week, however, Obama and President Lee Myung-bak of South Korea issued a joint statement saying that "the continuing commitment of extended deterrence, including the U.S. nuclear umbrella," provides assurance that the U.S. would respond if Pyongyang ever puts into action the threats it has repeatedly hurled at South Korea.

After meeting in the White House, Presidents Obama and Lee appeared in the Rose Garden, where the South Korean president said that "President Obama reaffirmed this firm commitment to ensuring the security of South Korea through extended deterrence, which includes the nuclear umbrella." Obama did not mention this commitment during his remarks.

Neither the White House press secretary, Robert Gibbs, nor the White House press corps raised the issue of the apparent shift in existing policy.

Before arriving in Washington, South Korean officials told the South Korean press that Lee would ask Obama for a written guarantee that the U.S. would use nuclear weapons against North Korea in the event of a North Korean nuclear attack. Evidently, he got what he asked for in the joint statement.

The North Koreans responded obliquely, the official Korean Central News Agency saying: "It is clear to anyone that the situation of the Korean Peninsula will grow more acute and the danger of outbreak of a nuclear war further increase in case the U.S. commitment to 'providing combat force for extended nuclear deterrence' to South Korea is documented."

KCNA called Lee the leader of "a gang of worst traitors as it seeks only to realize its scenario for invasion of the DPRK (Democratic People's Republic of Korea) with the help of foreign forces and maintain its power without caring about nuclear disaster to be imposed on the nation."

The U.S. nuclear commitment to South Korea may set a precedent for other allies concerned that Obama, having announced that he would seek a world free of nuclear weapons, may remove the U.S. nuclear umbrella over them. In particular, some Japanese political leaders and commentators have asserted that the U.S. can no longer be trusted to defend Japan against nuclear attack. Tokyo may thus ask for written commitment similar to that given to South Korea.

In a related development, the U.S. Air Force, which fields most of America's nuclear arms, has just revised its doctrine on nuclear operations in a document that will feed into the Pentagon's nuclear posture review now under way. During the Cold War, the Air Force said U.S. policy was "based on the threat of retaliation (and) served as the foundation for what is now called extended deterrence."

Today, the revised doctrine said, "extended deterrence is less about retaliation and more about posturing to convince an enemy that they are unlikely to achieve the political and military objectives behind any attack on the U.S. or one of our allies."

"Through alliances and treaties," the doctrine continued, without naming specific allies, "our extended deterrence strategy provides a nuclear umbrella to friendly and allied nations. Our nuclear umbrella assures allies of our commitment to their security and serves as a nonproliferation tool by obviating their need to develop and field their own nuclear arsenals."

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