Nuclear deterrence is taking center stage in Japan's security policy after Russia's threat to use nuclear weapons in its war against Ukraine, with Tokyo re-emphasizing the centrality of the U.S. nuclear umbrella amid regional tensions heightened by China's muscle flexing.

But Japan will still face a delicate juggling act since, as the only country to have experienced the horrors of a nuclear attack, it remains committed to leading discussions on bringing about a world without nuclear weapons and will continue to face calls from survivors of the 1945 atomic bombings to do more to bring that about.

When launching an attack on Ukraine in February, Russian President Vladimir Putin reminded the world that Russia remains one of the most powerful nuclear states, saying there should be no doubt that "any potential aggressor will face defeat and ominous consequences should it directly attack our country."

Japanese Prime Minister Fumio Kishida (2nd from L) visits the Ground Self-Defense Force's Asaka base straddling Tokyo and Saitama Prefecture on Nov. 27, 2021. (Pool photo) (Kyodo) ==Kyodo

A few days after the invasion was launched on Feb. 24, Putin put Russia's nuclear forces on high alert, making it harder for the United States and other members of the North Atlantic Treaty Organization to forcefully intervene in the war.

It has long been assumed that fear of a nuclear holocaust would prevent nuclear weapons from being used again, but some experts say Putin's nuclear threat has broken a taboo around the ultimate weapon.

Russia has been waging conventional warfare in its Ukraine invasion so far but it could be contemplating the use of tactical nuclear weapons, which are less potent than strategic nuclear arms and are intended to be used in limited strikes, some believe.

In East Asia, China has long held a no-first-use policy regarding its nuclear weapons, but whether it will stick to the policy has become unclear as the country ramps up its nuclear capabilities by modernizing and increasing the number of weapons in its arsenal.

China has 350 nuclear warheads, according to the Stockholm International Peace Research Institute. However, China will likely have at least 1,000 by 2030, according to a U.S. Defense Department estimate.

China is intensifying its military and diplomatic pressure on Taiwan, regarding the democratically governed island as a renegade province that must be reunified with the mainland, by force if necessary.

Beijing is also pressing its territorial claims in regional waters, including in the East China Sea where official Chinese vessels have repeatedly intruded into waters around the Senkaku Islands, which are administered by Japan but claimed by China.

The Dongfeng 41, a nuclear-capable intercontinental ballistic missile, is on display during a military parade in Beijing on Oct. 1, 2019, marking the 70th anniversary of the founding of the communist People's Republic of China. (Kyodo) ==Kyodo

North Korea, meanwhile, is pressing ahead with its development of intercontinental ballistic missiles and submarine-launched ballistic missiles while it is rumored to be preparing for a first nuclear test since 2017.

Seeing Russia make its nuclear threat, both China and North Korea have "reaffirmed the important role nuclear weapons play," said Ken Jimbo, an expert on international security at Keio University.

And that means not just diminished prospects for disarmament steps from China or denuclearization by North Korea, but also the increasing likelihood that either country could respond to a crisis by threatening a nuclear war, he said.

Japan and the United States, for their part, recently reaffirmed the importance of ensuring that "extended deterrence" -- U.S. commitment to the defense of Japan using a range of weapons capabilities, including nuclear -- remains credible and resilient.

But given the potentially enormous risks that extended deterrence entails, there is a persistent question about whether a country under the nuclear umbrella of an ally can trust the ally's commitment. And experts point to a growing possibility that countries such as China and North Korea could try to exploit this uncertainty to drive a wedge in the alliance.

To avoid having to debate whether to wage a nuclear war over, say, the Senkakus, there needs to be an arrangement to cope with regional-level conflicts through non-nuclear means, says Yoko Iwama, an international security expert at Japan's National Graduate Institute for Policy Studies.

"Japan must make every effort not to put the U.S. president in the dilemma of putting millions of people in Washington or New York at risk," Iwama said, stressing the need for a defense capability that can respond to "various levels of escalation."

A launch test of a Minuteman 3 intercontinental ballistic missile, capable of carrying nuclear warheads, is conducted at a U.S. base in California in May 2017. (Photo courtesy of the U.S. Air Force)(Kyodo)

Both Jimbo and Iwama say that in order to ensure that deterrence remains credible, Japan needs to show greater commitment, such as by increasing its defense budget.

The Japanese government is indeed moving in that direction, with Prime Minister Fumio Kishida pledging to "fundamentally reinforce" defense capabilities in the next five years with a "substantial increase" in the defense budget.

The renewed focus on nuclear deterrence, however, is likely to come at the expense of disarmament efforts, an area on which Japan has pledged to lead global discussions as the only country to have suffered atomic bombings.

Japan did not send a delegation to a recently held meeting of parties to the U.N. Treaty on the Prohibition of Nuclear Weapons in the Austrian capital Vienna.

Amid criticism that Japan did not even participate in the meeting as an observer, Kishida said that Japan "should work on realistic nuclear disarmament and non-proliferation efforts while maintaining a relationship of trust with the United States, our only alliance partner."

Security experts like Jimbo go so far as to say that given the proliferation of nuclear weapons, promoting the global norms of disarmament now appears to be an ineffective strategy.

But Iwama says deterrence and disarmament can and should go hand in hand, and that Japan should continue to play its role in disarmament efforts.

"Ultimately nuclear deterrence and disarmament share the same goal of pursuing stable order, and we must use both," she said, adding that there also needs to be an effort to "strengthen the sense of values that it is wrong to use nuclear weapons."