On December 18, the international NGO 'Peace Boat' announced that it, along with 19 other NGOs and civic organizations, sent a letter to Japanese Prime Minister Sanae Takaichi calling for immediate diplomatic efforts to halt the attacks on Iran by the United States and Israel. This request coincides with the upcoming Japan-U.S. summit on December 19, during which representatives from 14 organizations held an online press conference to address the situation.

The letter criticized the attacks as violations of international law and the UN Charter, asserting that Japan's potential support for military actions, such as deploying Self-Defense Forces or financing military expenditures, lacks legal justification under the Constitution. The groups stressed that providing any form of assistance poses significant risks in the region and should be avoided. Tachi Suzuki, a visiting professor at Nagasaki University and one of the signatories, warned that Japanese military cooperation could make U.S. bases in Japan targets for retaliation.

Moreover, Masakazu Yasui, Secretary-General of the Japan Council Against A- and H-Bombs (Gensuikyo), highlighted the paradox of the U.S. and Israel, both nuclear-armed states, planning to launch a preemptive strike against Iran under the pretext of its nuclear development. Yasui remarked that military force cannot resolve nuclear issues and urged both countries to abolish nuclear weapons as part of Japan's responsibility as a nation that has experienced atomic bombings.