The Japanese government faces a legal challenge to its decision to expand the scope of the military, a divisive shift for the pacifist nation that sparked protests at home and drew sharp criticism from China.
The legal action filed with the Tokyo District Court seeks to block a decision by Prime Minister Shinzo Abe's cabinet last week to reinterpret rules which have long banned the use of armed force except in narrowly defined circumstances.
Tokinao Chindo, 75, a former civil servant, said the move violated Japan's pacifist constitution, prompting his writ, believed to be the first such action.
"I hope other Japanese people will follow suit and file legal actions nationwide," Chindo said.
Following the change, Japanese troops would be able to come to the aid of allies, primarily the United States, if they come under attack from a common enemy, even if Japan is not the object of the attack.
Abe's move has caused anger at home, where the pacifism on which the constitution is built is an article of faith for many.
At least half the population opposes a more aggressive military stance, according to recent newspaper polls.
Following the historic decision, public support for Abe's cabinet slipped to below 50 per cent for the first time since he swept to power in 2012.
Chindo, who experienced the second world war as a small child, said: "I want to pass on the message 'never wage war' to the next generation."
"The Japanese shouldn't forget, just 69 years after the last war ended, that millions died during the war, and based on their sacrifices we embraced the Article 9" of the constitution, he added.
Building up military power would not work in safeguarding Japan, and would only lead to an arms race, Chindo said.
There was no immediate government reaction on the suit, but Chief Cabinet Secretary Yoshihide Suga reiterated that the military shift was too small to require a revision to the constitution.
"The constitution permits taking minimum necessary defence measures," he said.
Elsewhere, Mitsushige Yamanaka, the mayor of Matsuzaka, said this month he planned to launch a grass-roots movement to pursue a similar lawsuit.