Japan and South Korea Rethink U.S. Nuclear Umbrella Amid Trump’s Unpredictability

At an 18th-century Georgian manor house outside London, Japanese lawmaker Rui Matsukawa first began to question America’s reliability as her country’s ultimate security guarantor.

Matsukawa, a former deputy defense minister and member of the ruling Liberal Democratic Party’s national security council, attended a high-level bilateral conference at Fordham Abbey in March. There, she said, discussions with British lawmakers, diplomats, and business leaders underscored a seismic shift in global thinking: as U.S. President Donald Trump tilted toward Russia and chastised European allies, Europe was awakening to the need to shoulder more responsibility for its own security.

For Matsukawa, the lesson was clear: the same holds true for Japan, host to the largest overseas contingent of U.S. troops. “You can’t really take the U.S. presence for granted,” she told Reuters.

Matsukawa is part of a growing circle of senior Japanese lawmakers quietly entertaining an unthinkable idea in the only country ever devastated by atomic bombs: that Japan may one day need to consider nuclear weapons of its own. “Trump is so unpredictable, which is maybe his strength, but we always need a Plan B,” she said in her Tokyo office. “Plan B is maybe go independent — and then go nukes.”

Regional Reverberations

Trump’s approach is also unsettling South Korea, another nation shielded under America’s nuclear umbrella. Polls show as many as 75% of South Koreans support developing indigenous nuclear arms. The election of President Lee Jae Myung in June has muted some of the overt debate, but even within his Democratic Party, voices are emerging in favor of pursuing “nuclear latency” — the capability to build weapons quickly if needed.

Japan’s public remains more cautious, though attitudes are shifting. Interviews with lawmakers, officials, and retired military officers reveal a growing willingness to revisit the “Three Non-Nuclear Principles” — Japan’s decades-old policy not to produce, possess, or host nuclear weapons. Surveys also show rising public openness to rethinking the stance, even in Hiroshima, where survivors of the 1945 bombing once forged a national consensus around disarmament.

Strains in the Alliance

Allies’ anxiety stems from Trump’s “America First” agenda, his skepticism of NATO, imposition of tariffs on partners including Japan and South Korea, and controversial remarks about potentially absorbing Canada into the U.S. While the White House insists there has been “no change” in U.S. security commitments, many Asian officials privately voice doubts.

Japan’s Foreign Ministry insists Washington’s assurances remain “unwavering,” and the Defense Ministry says it has “full trust” that the U.S. will use “all types of capabilities, including nuclear forces,” to uphold its treaty obligations. Seoul echoed this sentiment, describing its alliance with Washington as the “foundation” of stability on the Korean Peninsula.

China, for its part, rejected claims of a growing nuclear threat, reaffirming its no-first-use pledge and accusing critics of attempting to “smear and defame” Beijing.

Rethinking the Nuclear Taboo

Eighty years after Hiroshima and Nagasaki, Japan has moved cautiously away from its postwar pacifism, rebuilding its military and enhancing U.S. security cooperation. Behind closed doors, Tokyo and Washington have in recent years expanded discussions on “extended deterrence” — how Japan’s intelligence, surveillance, and conventional strike capabilities might support the U.S. nuclear mission. These sensitive exchanges, revealed by former U.S. officials, have included scenarios on missile defense, counter-strike roles, and coordination in nuclear emergencies.

Now, senior members of Japan’s ruling LDP are weighing whether the non-nuclear principles could be revised or reinterpreted. They point out that the policy is neither enshrined in law nor legally binding. Matsukawa cited last year’s high-profile deployment of a U.S. nuclear-capable submarine to South Korea as an example Japan might emulate to reinforce deterrence.

For Matsukawa and others, the debate is no longer hypothetical. As faith in U.S. commitments wavers, Asia’s nuclear taboo is being tested as never before.