Flashpoints | Security | East Asia

Iran-Israel-US War Tests Japan’s Evolving Appetite for Collective Self-Defense

Can Japan send the SDF to Hormuz? The key will be how Tokyo classifies the current situation – but that is complicated by the conflict’s origin in a preemptive U.S. attack.

Arleigh Burke-class guided-missile destroyer USS Delbert D. Black (DDG 119) fires a Tomahawk Land Attack Missile (TLAM) in support of Operation Epic Fury, Feb. 28, 2026.

When Japan reinterpreted its constitution and updated its defense legislation under then-Prime Minister Abe Shinzo to allow for collective self-defense in 2015, there was much speculation about what the Self-Defense Forces (SDF) could now do. Following Prime Minister Takaichi Sanae’s remarks about Japan’s involvement in a Taiwan Strait contingency last November, these discussions regained salience. 

Now that U.S. President Donald Trump has asked Japan – alongside other U.S. allies – to send warships to reopen the Strait of Hormuz, there’s an urgent real-world stress test for Japan’s definition of collective self-defense. Although many have pointed out that legal, bureaucratic, and technical challenges remain, the political challenges are at least as daunting. 

The key issue at stake is how Japan classifies a given scenario. An “Important Influence Situation” (IIS) is defined as a situation “that has an important influence on Japan’s peace and security including situations that could result in a direct armed attack on Japan if left unattended.” Japan can respond to an IIS with logistics support activities, including “provision of ammunition” and “refueling and maintenance of aircraft preparing for combat operations” but not the provision of weapons. Japan can only implement response measures in an IIS in foreign territories with the consent of the foreign country concerned. To not accidentally become embroiled in other countries’ use of force, Japan “does not implement support activities in the areas where combat is actually taking place,” Japan’s Ministry of Defense noted in its 2025 White Paper. 

An “Armed Attack Situation” (AAS) is defined as a situation “where an external armed attack against Japan occurs or in which it is recognized that clear danger of an external armed attack against Japan is imminent,” and an “Anticipated Armed Attack Situation” (AAAS) is defined as a situation “that is not yet an armed attack situation but in which circumstances are critical and an armed attack against Japan is anticipated.” A “Survival-Threatening Situation” (STS), meanwhile, is defined as a situation “where an armed attack against a foreign country that is in close relationship with Japan occurs, which as a result, threatens Japan’s survival and poses a clear danger of fundamentally overturning Japanese people’s right to life, liberty, and pursuit of happiness.” 

If it is determined that the use of force is “necessary” to respond to a situation that has been classified as an AAS, AAAS, or STS, and “there are no other appropriate means available to ensure Japan’s survival and protect its people,” then and only then will the prime minister issue a Defense Operation order to the whole or part of the SDF. In other words, a declaration of an STS is the only way Japan can mobilize the SDF for defense operations under its right of collective self-defense (mobilizing the SDF under an AAS/AAAS would fall under the right to individual self-defense).

That is the legal morass that must be navigated by a political compass. And politics certainly matter. For example, in declaring an STS, a key phrase is “a foreign country that is in close relationship with Japan.” Countries are not legally classified as meeting this criteria or not; it is a political determination made by the sitting prime minister. During the debates leading up to revision in 2015, it was widely understood that this referred to the United States. 

Takaichi confirmed this stance last December when she admitted that “the possibility of countries other than the United States qualifying is quite limited in reality.” But she kept Japan’s future options as open as possible by stating that whether, for example, Taiwan would fall under this category “is not predetermined and would be judged based on individual and specific situations.”   

Takaichi went off-script last November in not refraining from answering questions about hypothetical scenarios – in that case regarding Taiwan. However, some made the argument that her answer – “If naval ships are used in order to use force, that would be a case that can only be considered a survival-threatening situation” – was not a radical departure from Japan’s past policy. 

The quiet part that Takaichi said out loud was that it would be almost impossible for Japan to sit out any Taiwan war that the United States decides to get involved in. For one thing, there are U.S. military bases in Japan – and a high likelihood that China would attack those bases if the United States were to mobilize for Taiwan’s defense. Then there’s the strong political consensus in Japan that the United States falls under the definition of “a foreign country that is in close relationship with Japan.”

While there is political consensus that failing to defend the United States under certain circumstances could threaten Japan’s survival, given how dependent Japan is on the U.S. alliance for its defense, there is not yet a consensus around specific scenarios. Abe and Takaichi........

© The Diplomat