Explained | How Trump's Hormuz push tests Japan's pacifist limits

Tokyo: US President Donald Trump's call for allies to send warships ​to escort oil tankers through the Strait of Hormuz has revived questions about how far pacifist Japan can go to support its closest ally in a conflict.

Here are the narrow legal options available to Prime Minister Sanae Takaichi and the past precedents that could shape her decision.

Policing role

After its defeat ⁠in World War Two, Japan adopted a US-drafted constitution that renounced the use of force to settle international disputes.

Within those limits, however, Takaichi ⁠can deploy Maritime Self-Defense Force vessels on law-enforcement operations overseas. The clearest example is an anti-piracy mission ‌off Somalia and in the Gulf of Aden, ​which Japan joined in 2009 after revising legislation to allow Japanese warships to protect vessels of all nationalities.

Following Trump's request, Defence Minister Shinjiro Koizumi told parliament on Monday that a similar policing action could be considered "if further measures by the SDF are deemed necessary."

But that framework is designed for policing, not combat. Applying it to operations where Japanese forces could confront a state actor such as Iran would be legally ​problematic.

Higher legal threshold

In a significant step back ⁠from its post-war pacifism, Japan passed security laws in 2015 allowing it to use force overseas in limited circumstances. That is permitted only if an attack, including on a close security partner, threatens Japan's survival and no other means are available to address it.

The laws permit a broader use of force than possible in anti-piracy operations, but the legal threshold ⁠for invoking them is ‌far higher. Takaichi would need to argue that the disruption to energy supplies caused by the closure of the Strait of Hormuz constituted an existential threat, a case likely to face stiff political and ‌public opposition.

The legislation has never been used and Takaichi this week said Japan would prioritize diplomatic efforts to de-escalate tensions in the Middle East.

Past deployments

Past Japanese operations ⁠in and around the Middle East offer Takaichi a guide, and underscore how Tokyo has kept within its legal limits.

During the 1991 Gulf War, Japan contributed money rather than personnel, a ‌decision criticised by the US and other nations as 'chequebook diplomacy'. After hostilities ended, it dispatched minesweepers to the ‌Persian Gulf, marking the SDF's first ever overseas deployment.

"Japan's poor response during the Gulf War remains a ‌scar in the national consciousness. So I suspect her (Takaichi's) government is looking hard for some way to show the flag," ‌said Michael Green, a professor and chief executive of The United States Studies Centre at the University of Sydney.

After the September 11 attacks in 2001, Japan sent MSDF vessels to the Indian Ocean to refuel and support US-led operations in Afghanistan. That mission ran for eight years but did not ⁠involve combat or escort ⁠missions.

In 2004, Japan sent about 600 ground troops to Iraq for reconstruction work, along with aircraft to help transport supplies and personnel. The soldiers were only allowed to use force as a last resort, and were guarded by Dutch and Australian troops during their two-year stint.

In 2019, after attacks on tankers that Washington blamed on Iran, Japan diverted a destroyer and patrol plane from anti-piracy operations near Somalia to gather intelligence in the Gulf of Oman, the Arabian Sea and Gulf of Aden. However, they stayed outside the Strait of Hormuz and the Persian Gulf.

International law problem

Japan also faces a separate legal question: whether the ​U.S. military action is in line with international law.

Under the United Nations Charter, the use of force is generally prohibited ​unless authorised by the UN Security Council or justified ​as self-defence against an armed attack.

For a country that has long been a staunch supporter of international law, that uncertainty could further limit how far Tokyo is ​willing to go.

Legal experts are divided over whether the U.S. strikes on Iran meet that threshold and Takaichi has so far declined to say what Japan's position is.