As the security environment has deteriorated in East Asia, Japan's dependency on the U.S. nuclear umbrella and the bilateral "nuclear bond" have been strengthened through under-the-table discussions, according to a Japanese government memo recently obtained by Kyodo News and interviews with officials.

In February, the administration of President Donald Trump unveiled its nuclear policy guideline "Nuclear Posture Review," emphasizing the U.S. security commitment to defend allies including Japan which is under nuclear threat by North Korea.

"Potential adversaries must understand that the United States has the will and response options necessary to deter nuclear attack under any conditions...our deterrence strategy for North Korea makes clear that any North Korean nuclear attack against the United States or its allies and partners is unacceptable and will result in the end of that regime," the NPR says.

It also proposes newly introducing low-yield nuclear warheads for submarine-launch ballistic missile and sea-launch cruise missile to boost credibility of the U.S. nuclear deterrence.

A day after the release of the policy guideline, Japanese Foreign Minister Taro Kono issued a statement, saying, "Japan highly appreciates the latest NPR which clearly articulates the U.S. resolve to ensure the effectiveness of its deterrence and its commitment to providing extended deterrence to (Japan)."

"We couldn't help issuing this statement because this NPR adopted all we have wanted," a senior official of the Japanese Foreign Ministry said.

Also, one secret memo written by Japanese strategists nine years ago strongly indicates why the administration of Prime Minister Shinzo Abe gives such a high evaluation to the latest NPR.

"It is difficult for us to specify the weapon systems which the United States should maintain or acquire. But we can list, though not exhaustively, several desired characteristics which the U.S. deterrence capability should have," says the three-page memo obtained by Kyodo News and which elaborates on what kind of nuclear capabilities Japan has wanted.

The memo dated Feb. 25, 2009, was presented to the U.S. Congressional Commission on U.S. Strategic Posture which was composed of 12 nuclear experts and chaired by former Defense Secretary William Perry. A main purpose of the bipartisan commission was to make concrete proposals for the administration of then President Barack Obama which was drafting its own NPR.

The memo was explained to the commission by Takeo Akiba, then Japanese political minister to Washington. Akiba is now vice foreign minister, a top bureaucrat at the Foreign Ministry.

The memo listed six "desired characteristics of U.S. deterrence capabilities" such as "flexible capabilities, credible capabilities, prompt capabilities, discriminating and selective capabilities, stealthy/demonstrable capabilities, and sufficient capabilities."

"The U.S. possible unilateral reduction of its operationally deployed strategic nuclear warheads may have an adverse effect on Japan's security. When the United States engages in nuclear reduction talks with Russia, China's nuclear expansion and modernization should always be borne in mind," the memo says.

In 2009, Obama took up the U.S. presidency and declared the United States would "seek the peace and security of a world without nuclear weapons."

During a recent interview with Kyodo News, a Japanese official involved in the drafting process of the 2009 memo said Japanese security policy elites like Akiba were so concerned about Obama's "nuclear-free world" vision that they intensively lobbied influential U.S. experts for reversing the course set by the then president.

The memo presented by Akiba also emphasized that the U.S. deterrence capabilities should be "flexible enough to hold a wide variety of adversary threats at risk." It raised "cyberattack" as one of the "adversary threats" to be targeted by U.S. nuclear arsenals.

The NPR under the Trump administration indicates cyberattack, categorized as "non-nuclear strategic attack," would be a potential nuclear target. Also, a concept of introducing low-yield weapons, which was proposed in the latest NPR, is one of the items Japanese security experts view as important as suggested by the secret memo in 2009.

Since 2010, U.S. and Japanese security officials have periodically conducted a series of secret talks on the allies' deterrence policy through a bilateral framework called "Extended Deterrence Dialogue."

"I have no surprise at the Trump NPR. It's written about the consensus of both nuclear communities in Japan and the United States," said one of the Japanese participants in the dialogue, who was also involved in the drafting process of the 2009 memo.