Washington (CNN) The U.S. and its closest Asian allies reaffirmed decades of security cooperation Thursday as their cornerstone alliance is being challenged by Republican presidential front-runner Donald Trump.

Trump's statements scrapping years of U.S. consensus on nuclear issues have prompted discomfort and disbelief from U.S. officials and experts, while foreign governments have challenged his ideas.

Meeting with leaders of Japan and South Korea ahead of the Nuclear Security Summit in Washington, President Barack Obama said that the three Pacific nations aim to "restore a sense of stability and peace to the region."

The meeting looked to reinforce a security alliance that has stabilized Asia for decades and now faces new challenges in an assertive China and increasingly erratic North Korea.

More broadly, the White House hopes to further its goal to eliminate -- or at least reduce -- the world's stockpile of nuclear weapons and material. It's a position that Republican and Democratic administrations have held since World War II.

But Trump is ripping up the establishment playbook on foreign policy, just has he has defied the traditional storyline of the presidential campaign, repeatedly breaking rules only to emerge -- so far -- relatively unscathed.

The Republican front-runner has said that he sees nuclear proliferation as the world's biggest challenge, but he's also declared that security agreements in Europe and the Middle East might have to be redrawn. And he's suggested that Japan and South Korea may need to step out from under the U.S. security umbrella and develop their own nuclear arsenal.

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"At some point we have to say, you know what, we're better off if Japan protects itself against this maniac in North Korea," Trump told Anderson Cooper at a CNN town hall in Milwaukee on Tuesday. "We're better off, frankly, if South Korea is going to start to protect itself."

Trump's proposals on arming Asia drew censure from the administration, which has made disarmament a legacy issue for the President.

Obama has focused on nuclear dangers since his days as a U.S. senator, writing anti-proliferation legislation with Indiana's Republican Sen. Richard Lugar. His first congressional trip abroad was to inspect nuclear weapons facilities in Eastern Europe and Russia.

"Frankly, it would be catastrophic were the United States to shift its position and indicate that we somehow support proliferation of nuclear weapons to additional countries," Deputy National Security Advisor Ben Rhodes said Thursday when asked about Trump's outlook.

"The entire premise of American foreign policy as it relates to nuclear weapons for the last 70 years has been focused on preventing the proliferation of nuclear weapons to additional states," Rhodes said. "That's been the position of bipartisan administrations, everybody who's occupied the Oval Office."

Japanese and U.S. officials said Trump's suggestion didn't come up in the meeting between South Korean President Park Geun-hye, Japanese Prime Minister Shinzo Abe and Obama, but Japanese leaders have addressed it publicly.

Japan's Prime Minister Shinzo Abe responded after the Republican candidate's comments Tuesday by saying, "whoever will become the next president of the United States, the Japan-U.S. alliance is the cornerstone of Japan's diplomacy."

Foreign Minister Fumio Kishida said more bluntly, "It is impossible that Japan will arm itself with nuclear weapons."

Analysts say Trump's proposal to have South Korea or Japan embrace nuclear weapons is so unrealistic that it won't even merit discussion around water coolers at the nuclear summit.

"I don't think there'll be a discussion among the leaders or even a hallway conversation," said Victor Cha, a senior fellow at the Center for Strategic and International Studies who was President George W. Bush's top advisor on North Korea.

"If so," Cha added, "only in jest, as people are going to refill their coffee cups."

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Trump's proposal would diverge from the region's history, particularly in Japan.

The only country to have been struck by nuclear bombs, in Hiroshima and Nagasaki, Japan has a vociferous anti-nuclear constituency. And while its military is beginning to take on a more assertive posture, most experts say the scars of Japanese aggression during World War II mean the country and the region feel more comfortable with Tokyo working under the broader U.S. security umbrella.

Trump has also suggested redrawing U.S. security relationships in other regions, arguing that Germany and Saudi Arabia need to do more in their own defense or shell out bigger payments to the U.S. for the protection it provides.

"They have to protect themselves or they have to pay us," Trump said in Milwaukee.

One Western diplomat who is watching the presidential race closely said the working theory is that Trump is negotiating and that, as in all negotiations, he is starting at the most extreme positions and would slowly work his way closer to the center once the general election campaign starts.

This diplomat, who spoke anonymously to discuss U.S. politics, said another theory is that if Trump does become the Republican nominee, experts who are advising his competitors will eventually start helping the real estate magnate.

Trump trampled on yet another foreign policy axiom on Wednesday, when he refused to rule out using nuclear weapons to put a swift and definitive end to the threat of ISIS.

"I would never take any of my cards off the table," Trump told an MSNBC Town Hall.

Asked by CNN whether any coalition members had suggested using nuclear weapons against ISIS, Major Gen. Doug Chalmers, a British deputy commander with the coalition, acknowledged surprise.

"I have never heard (it) discussed amongst any of our coalition members at any stage," he said. "Actually, I have to admit, that one has taken me completely by surprise. The simple answer is no."