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 are both frontrunners in trying to build more specific consensuses. 

In 2021, Abe – at the time speaking not as the prime minister of Japan but a Diet member and heavyweight member of the ruling Liberal Democratic Party (LDP) – said that “a Taiwan emergency is a Japanese emergency, and therefore an emergency for the Japan-U.S. alliance.” 

In April 2025, before her election to the LDP presidency and premiership, Takaichi said that because Japan relies heavily on maritime trade, any regional conflict threatening sea routes would pose an existential risk to the Japan. Speaking as Diet members, Abe and Takaichi strongly implied that an STS can be declared in a Taiwan Strait contingency.  

What about the Middle East and the ongoing Iran-Israel-U.S. war? Japan has not yet made a determination whether the conflict constitutes an STS. During the security legislation debate in 2015, Abe said an Iranian blockade of the Strait of Hormuz with mines would constitute an STS, making an SDF minesweeping operation legal. He argued that an STS could be declared because the blockade halting oil imports would endanger the lives of Japanese. However, he also argued during the 2015 Diet deliberations that Japan would not exercise the right to collective self-defense for a country that launched a preemptive attack. 

As a former senior Defense Ministry official told Yomiuri Shimbun, “We never anticipated a scenario in which the United States would launch a preemptive attack like this one. It may be difficult to apply the security-related legislation.” 

Similarly, the Japan-U.S. bilateral defense guidelines, also updated in 2015, refer to a Middle East scenario – but it presumed Iranian aggression, not a preemptive Israeli-U.S. strike. Again, this limits the applicability of the guidelines to the present situation.

Though Takaichi is likely relieved that U.S. President Donald Trump dropped his demand that Japan send warships to the Strait of Hormuz before their summit on March 19, the war is likely to drag on and U.S. pressure in the future for an SDF contribution is in the realm of the possible. 

How would Japan handle such requests? A decisive factor will be how the “situation” – the war – is classified.

Without a direct attack on Japan, an AAS or AAAS cannot be declared. Speaking about a damaged Mitsui O.S.K. Lines container ship, a Japanese government official stated, “At this point, there is no evidence to conclude Japan has been subject to armed attack by Iran.” 

Barring a direct Iranian attack on Japan, this leaves declaring an STS – with the United States as “a foreign country that is in close relationship with Japan” – as the only option for a Japanese defense operation during active combat between states. Absent the declaration of an STS, even removing mines before a ceasefire is declared could be interpreted as Japan using force against Iran – forbidden under Article 9 of the Japanese Constitution.  

Declaring an IIS would allow Japan to provide some forms of logistical, rear-area support outside the area of active combat. However, even declaring an IIS could be very political given that Japan has yet to declare any conflict as one of these four types of “situations” since the 2015 update. As of this writing, Japan has yet to declare the war an IIS.

Yet the sitting government is hesitant to speak so plainly – again reflecting not only the legal quagmire but the political nature of these determinations. Takaichi has said that Japan would refrain from making a legal assessment of whether the Israeli-U.S. attack was a measure for self-defense or not. She has not questioned the legality of the U.S. strikes and is not expected to do so during her summit meeting with Trump. 

However, the Japanese public has a clear position. an Asahi Shimbun survey found that only 9 percent of respondents supported the Israeli-U.S. war on Iran, while 82 percent of respondents do not support it. Furthermore, 51 percent did not approve of Takaichi’s ambiguity on the legality of the war while only 34 percent approved. 

What Japan can and cannot do in any given war is both a legal question and a political one. Though the Japanese government tried to prepare for various different types of wars, a war started by an illegal Israeli-U.S. attack was not one they had been preparing for. If the war drags on, a blockade of the Strait of Hormuz could reasonably be considered an STS or IIS. However, given how the war started, forging the political consensus among the Japanese people to do so would be incredibly challenging. 

Diet approval is necessary for both an IIS and STS designation. Without Diet approval, the SDF will not be able to be involved in rear-area, logistical support of the U.S. military (under an IIS designation) or defense operations alongside the U.S. military (under an STS designation). 

The world looks very different in 2026 than it did in 2015. Despite Abe’s confidence about the value of the Japan-U.S. alliance and the need for Japan to be able to “do more” to keep the United States committed to Japan’s defense, the Japanese government was wise to preserve enough political flexibility to be able to adjust the new reality today. Time will tell how the Japanese government uses the political flexibility that it has within the legal constraints.