
Editor’s Note: This is a guest post by Julian Ku, a law professor at Hofstra University, which responds to a recent Pacific Realist article that argued that Taiwan played a significant if unstated role in Japan’s move toward collective self-defense. This article first appeared on the always excellent Opinio Juris, a one-stop shop for insightful and timely analysis of IR and current events from an international legal perspective.

I’ve been swamped with various projects and distractions here in Taiwan (mostly food-related), so I didn’t notice until today this very interesting Zachary Keck post about how Japan’s recent decision to re-interpret its constitutional provision to allow expanded overseas military activities would enable Japan to help defend Taiwan against an attack from China. It’s a fascinating post, but it also made me think of an interesting wrinkle that cuts against his argument. It is almost certainly true that international law prohibits any military action by Japan (or the U.S.) to defend Taiwan from a Chinese attack.

In his post, Keck notes that Japan’s decision to reinterpret its constitution does NOT allow Japan to fully exercise its rights to collective self-defense under international law, but it does allow Japan to provide military support to allies where Japan itself is threatened. But he then argues that even under this more narrow “collective self-defense” right, Japan could (and probably would) intervene to assist Taiwan in a military defense against a Chinese invasion.

I think this could be right as a matter of Japanese constitutional law if an invasion of Taiwan could be plausibly construed as a threat to Japan, but there is a strange international law flaw to this argument. Under black-letter international law, Japan cannot use military force in Taiwan absent China’s consent, even if the Taiwan government requests its assistance. Why? Because the UN Charter’s Article 51 only authorizes an act of “collective self-defense if an armed attack occurs against a Member of the United Nations.” Taiwan is not a member of the United Nations, and to make matters worse from Taiwan’s perspective, Japan recognizes the government in Beijing as the rightful government of China, and Japan further recognizes that Taiwan is part of China.

So unless Japan is able to plausibly claim that an attack on Taiwan triggers Japan’s own inherent self-defense right (and I think this is a non-starter as a legal argument), and unless a Chinese invasion could be said to justify humanitarian intervention (another very difficult argument), Japan would violate the U.N. Charter if it used military force in a way that violated the territorial integrity of another UN member (China). Japan could not invoke its collective self-defense rights unless it recognized Taiwan as an independent nation. And even that would probably not be enough to satisfy international law requirements, since Japan’s unilateral recognition of Taiwan as an independent state would not necessarily satisfy international law either. And good luck, Taiwan, getting U.N. membership.

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By the way, this analysis applies equally (or even with greater force) to the United States. The U.S. quasi-defense guarantee to Taiwan has it completely backwards (from a legal point of view):

If Taiwan declares independence, the U.S. has signaled it would not consider itself bound to defend Taiwan against a Chinese invasion. Yet that would be (at least in theory) one state (China) committing aggression against another state (Taiwan), and almost certainly illegal.

consider itself bound to defend Taiwan against a Chinese invasion. Yet that would be (at least in theory) one state (China) committing aggression against another state (Taiwan), and almost certainly illegal. If Taiwan keeps the status quo and does not declare independence, and China still invades, the U.S. has signaled that it would come to Taiwan’s defense. But that would be one state (China) using force within its own territory to put down secessionists (a la Ukraine) and almost certainly legal.

So the U.S. (and maybe Japan) are now committed to defend Taiwan only in a situation that would require the U.S. and Japan to violate the U.N. Charter. It’s international-law-bizarro world!


Of course, this bizarro-from-a-legal-point-of-view policy suits U.S. purposes, since it is the policy most likely to avoid military conflict with China. But it also reveals how use of force rules in the U.N. Charter have little relevance to shaping the behavior of the U.S., Japan (and probably China) in any conflict over Taiwan. Japan and the U.S. should (and probably are) ready to ignore these legal rules when making their determinations about whether to defend Taiwan. And all in all, that’s a good thing (especially while I am still here in Taipei!).